Develop

.github/ISSUE_TEMPLATE/custom.md

@@ -0,0 +1,10 @@
+---
+name: Custom issue template
+about: Describe this issue template's purpose here.
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+

.github/ISSUE_TEMPLATE/feature_request.md

@@ -2,7 +2,7 @@
 name: Feature request
 about: Suggest an idea for this project
 title: ''
-labels: ''
+labels: question
 assignees: ''
 
 ---

.gitignore

@@ -1,4 +1,6 @@
 .idea/
 .vscode/
-
+automate_git.bat
+auto_commit.sh
+auto_commit.bat
 *.iml

README.md

@@ -2,4 +2,7 @@
 Php-based LeetCode algorithm problem solutions, regularly updated.
 
 
-![Alt](https://repobeats.axiom.co/api/embed/9f698d3cdb218feb946714e458ac140a276b39a5.svg "Repobeats analytics image")
+![Repobeats analytics image](https://repobeats.axiom.co/api/embed/9f698d3cdb218feb946714e458ac140a276b39a5.svg "Repobeats analytics image")
+
+
+

algorithms/000040-combination-sum-ii/README.md

@@ -0,0 +1,56 @@
+40\. Combination Sum II
+
+**Difficulty:** Medium
+
+**Topics:** `Array`, `Backtracking`
+
+Given a collection of candidate numbers (`candidates`) and a target number (`target`), find all unique combinations in `candidates` where the candidate numbers sum to `target`.
+
+Each number in `candidates` may only be used **once** in the combination.
+
+**Note:** The solution set must not contain duplicate combinations.
+
+**Example 1:**
+
+- **Input:** candidates = [10,1,2,7,6,1,5], target = 8
+- **Output:** [[1,1,6], [1,2,5], [1,7], [2,6]]
+
+**Example 2:**
+
+- **Input:** candidates = [2,5,2,1,2], target = 5
+- **Output:** [[1,2,2], [5]]
+
+**Constraints:**
+
+- <code>1 <= candidates.length <= 100</code>
+- <code>1 <= candidates[i] <= 50</code>
+- <code>1 <= target <= 30</code>
+
+
+**Solution:**
+
+
+We can use a backtracking approach. The key idea is to sort the array first to handle duplicates easily and then explore all possible combinations using backtracking.
+
+Let's implement this solution in PHP: **[40. Combination Sum II](https://github.com/mah-shamim/leet-code-in-php/tree/main/algorithms/000040-combination-sum-ii/solution.php)**
+
+### Explanation:
+
+1. **Sorting**: The candidates array is sorted to handle duplicates easily and to ensure combinations are formed in a sorted order.
+2. **Backtracking**: The `backtrack` function is used to explore all possible combinations.
+   - If the `target` becomes zero, we add the current combination to the result.
+   - We iterate over the candidates starting from the current index. If a candidate is the same as the previous one, we skip it to avoid duplicate combinations.
+   - We subtract the current candidate from the target and recursively call the `backtrack` function with the new target and the next index.
+   - The recursive call continues until we either find a valid combination or exhaust all possibilities.
+3. **Pruning**: If the candidate exceeds the target, we break out of the loop early since further candidates will also exceed the target.
+
+This code will output all unique combinations that sum up to the target while ensuring that each candidate is used only once in each combination.
+
+**Contact Links**
+
+If you found this series helpful, please consider giving the **[repository](https://github.com/mah-shamim/leet-code-in-php)** a star on GitHub or sharing the post on your favorite social networks 😍. Your support would mean a lot to me!
+
+If you want more helpful content like this, feel free to follow me:
+
+- **[LinkedIn](https://www.linkedin.com/in/arifulhaque/)**
+- **[GitHub](https://github.com/mah-shamim)**

algorithms/000040-combination-sum-ii/solution.php

@@ -0,0 +1,44 @@
+<?php
+
+class Solution {
+
+    /**
+     * @param Integer[] $candidates
+     * @param Integer $target
+     * @return Integer[][]
+     */
+    function combinationSum2($candidates, $target) {
+        sort($candidates);
+        $result = [];
+        $this->backtrack($candidates, $target, 0, [], $result);
+        return $result;
+    }
+
+    /**
+     * @param $candidates
+     * @param $target
+     * @param $start
+     * @param $currentCombination
+     * @param $result
+     * @return void
+     */
+    function backtrack($candidates, $target, $start, $currentCombination, &$result) {
+        if ($target == 0) {
+            $result[] = $currentCombination;
+            return;
+        }
+
+        for ($i = $start; $i < count($candidates); $i++) {
+            if ($i > $start && $candidates[$i] == $candidates[$i - 1]) {
+                continue; // Skip duplicates
+            }
+
+            if ($candidates[$i] > $target) {
+                break; // No need to continue if the candidate exceeds the target
+            }
+
+            self::backtrack($candidates, $target - $candidates[$i], $i + 1, array_merge($currentCombination, [$candidates[$i]]), $result);
+        }
+    }
+
+}

algorithms/000075-sort-colors/README.md

@@ -2,6 +2,8 @@
 
 Medium
 
+**Topics:** `Array`, `Two Pointers`, `Sorting`
+
 Given an array `nums` with `n` objects colored red, white, or blue, sort them [in-place](https://en.wikipedia.org/wiki/In-place_algorithm) so that objects of the same color are adjacent, with the colors in the order red, white, and blue.
 
 We will use the integers `0`, `1`, and `2` to represent the color red, white, and blue, respectively.
@@ -24,4 +26,81 @@ You must solve this problem without using the library's sort function.
 - <code>1 <= n <= 300</code>
 - `nums[i]` is either `0`, `1`, or `2`.
 
-**Follow-up:** Could you come up with a one-pass algorithm using only constant extra space?
+**Follow-up:** Could you come up with a one-pass algorithm using only constant extra space?
+
+**Hint:**
+1. A rather straight forward solution is a two-pass algorithm using counting sort.
+2. Iterate the array counting number of 0's, 1's, and 2's.
+3. Overwrite array with the total number of 0's, then 1's and followed by 2's.
+
+
+**Solution:**
+
+
+To solve this problem, we can follow these steps:
+
+The goal is to sort the array `nums` with elements representing the colors red (0), white (1), and blue (2) in one pass, using constant extra space.
+
+### Approach:
+
+The most efficient way to solve this problem is by using the Dutch National Flag algorithm, which is a one-pass algorithm with constant extra space. The idea is to use three pointers:
+- `low` to track the position for the next 0 (red),
+- `mid` to traverse the array,
+- `high` to track the position for the next 2 (blue).
+
+### Algorithm:
+1. Initialize three pointers:
+   - `low` at the start (0),
+   - `mid` at the start (0),
+   - `high` at the end (n-1) of the array.
+
+2. Traverse the array with `mid`:
+   - If `nums[mid]` is `0` (red), swap `nums[mid]` with `nums[low]`, then increment both `low` and `mid`.
+   - If `nums[mid]` is `1` (white), move `mid` to the next element.
+   - If `nums[mid]` is `2` (blue), swap `nums[mid]` with `nums[high]` and decrement `high`.
+
+3. Continue until `mid` exceeds `high`.
+
+This algorithm sorts the array in-place with a single pass.
+
+Let's implement this solution in PHP: **[75. Sort Colors](https://github.com/mah-shamim/leet-code-in-php/tree/main/algorithms/000075-sort-colors/solution.php)**
+
+```php
+<?php
+// Example usage:
+$nums1 = [2, 0, 2, 1, 1, 0];
+sortColors($nums1);
+print_r($nums1); // Output: [0, 0, 1, 1, 2, 2]
+
+$nums2 = [2,0,1];
+sortColors($nums2);
+print_r($nums2); // Output: [0,1,2]
+
+?>
+```
+
+### Explanation:
+
+1. **Initialization:**
+   - `low` starts at the beginning (`0`).
+   - `mid` starts at the beginning (`0`).
+   - `high` starts at the end (`n-1`).
+
+2. **Loop Through Array:**
+   - If `nums[mid]` is `0`, it swaps with `nums[low]` because `0` should be at the front. Both pointers `low` and `mid` are then incremented.
+   - If `nums[mid]` is `1`, it is already in the correct position, so only `mid` is incremented.
+   - If `nums[mid]` is `2`, it swaps with `nums[high]` because `2` should be at the end. Only `high` is decremented, while `mid` stays the same to check the swapped element.
+
+3. **Completion:**
+   - The loop continues until `mid` passes `high`, ensuring that the array is sorted in the order of `0`, `1`, and `2`.
+
+This approach is optimal with O(n) time complexity and O(1) space complexity.
+
+**Contact Links**
+
+If you found this series helpful, please consider giving the **[repository](https://github.com/mah-shamim/leet-code-in-php)** a star on GitHub or sharing the post on your favorite social networks 😍. Your support would mean a lot to me!
+
+If you want more helpful content like this, feel free to follow me:
+
+- **[LinkedIn](https://www.linkedin.com/in/arifulhaque/)**
+- **[GitHub](https://github.com/mah-shamim)**

algorithms/000075-sort-colors/solution.php

@@ -6,22 +6,29 @@ class Solution {
      * @param Integer[] $nums
      * @return NULL
      */
-    function sortColors(array &$nums) {
-        $l = 0;                // The next 0 should be placed in l.
-        $r = count($nums) - 1;  // The next 2 should be placed in r.
+    function sortColors(&$nums) {
+        $low = 0;
+        $mid = 0;
+        $high = count($nums) - 1;
 
-        for ($i = 0; $i <= $r;) {
-            if ($nums[$i] == 0) {
-                list($nums[$i], $nums[$l]) = array($nums[$l], $nums[$i]);
-                $i++;
-                $l++;
-            } elseif ($nums[$i] == 1) {
-                $i++;
-            } else {
-                // We may swap a 0 to index i, but we're still not sure whether this 0
-                // is placed in the correct index, so we can't move pointer i.
-                list($nums[$i], $nums[$r]) = array($nums[$r], $nums[$i]);
-                $r--;
+        while ($mid <= $high) {
+            if ($nums[$mid] == 0) {
+                // Swap nums[low] and nums[mid]
+                $temp = $nums[$low];
+                $nums[$low] = $nums[$mid];
+                $nums[$mid] = $temp;
+
+                $low++;
+                $mid++;
+            } elseif ($nums[$mid] == 1) {
+                $mid++;
+            } else { // nums[$mid] == 2
+                // Swap nums[mid] and nums[high]
+                $temp = $nums[$high];
+                $nums[$high] = $nums[$mid];
+                $nums[$mid] = $temp;
+
+                $high--;
             }
         }
     }

algorithms/000078-subsets/README.md

@@ -1,6 +1,8 @@
 78\. Subsets
 
-Medium
+**Difficulty:** Medium
+
+**Topics:** `Array`, `Backtracking`, `Bit Manipulation`
 
 Given an integer array `nums` of **unique** elements, return _all possible subsets[^1] (the power set)._
 
@@ -23,4 +25,82 @@ The solution set **must not** contain duplicate subsets. Return the solution in
 - <code>-10 <= nums[i] <= 10</code>
 - All the numbers of `nums` are **unique**.
 
-[^1]: **Subset** <code>A **subset** of an array is a selection of elements (possibly none) of the array.</code>
+[^1]: **Subset** <code>A **subset** of an array is a selection of elements (possibly none) of the array.</code>
+
+**Solution:**
+
+
+To solve this problem, we can follow these steps:
+
+Let's implement this solution in PHP: **[78. Subsets](https://github.com/mah-shamim/leet-code-in-php/tree/main/algorithms/000078-subsets/solution.php)**
+
+```php
+<?php
+// Test the function with example inputs
+print_r(subsets([1,2,3])); // Output: [[],[1],[2],[1,2],[3],[1,3],[2,3],[1,2,3]]
+print_r(subsets([0])); // Output: [[],[0]]
+?>
+```
+
+### Explanation:
+
+1. **Function Signature and Input**:
+   - The function `subsets()` takes an array of integers `nums` as input and returns an array of arrays, where each inner array represents a subset of `nums`.
+   - The return type is `Integer[][]`, meaning an array of arrays of integers.
+
+2. **Initialization**:
+   - `$ans = [[]];`: Initialize the result array `$ans` with an empty subset (`[]`). This is the base case where the subset is empty.
+
+3. **Iterating Over Each Number**:
+   - `foreach($nums as $num)`: Loop through each number in the input array `nums`. For each number `num`, you'll generate new subsets by adding `num` to each of the existing subsets.
+
+4. **Generating New Subsets**:
+   - `$ans_size = count($ans);`: Before modifying the `$ans` array, store its current size in `$ans_size`. This is necessary to prevent an infinite loop as new subsets are added.
+   - `for($i = 0; $i < $ans_size; $i++)`: Iterate over the current subsets stored in `$ans` up to `$ans_size`.
+      - `$cur = $ans[$i];`: Take the current subset.
+      - `$cur[] = $num;`: Append the current number `num` to this subset.
+      - `$ans[] = $cur;`: Add this new subset back into `$ans`.
+
+5. **Return the Result**:
+   - `return $ans;`: After processing all numbers in `nums`, return the final array of all possible subsets.
+
+### Example Execution:
+
+Let's go through an example to see how it works.
+
+- **Input**: `$nums = [1, 2]`
+- **Initial State**: `$ans = [[]]`
+
+**Iteration 1 (`num = 1`)**:
+- `$ans_size = 1`: (Because there's only one subset, `[]`)
+- For `i = 0`:
+   - `$cur = []`: Take the empty subset.
+   - `$cur[] = 1`: Append `1` to it, resulting in `[1]`.
+   - `$ans = [[], [1]]`: Add `[1]` to `$ans`.
+
+**Iteration 2 (`num = 2`)**:
+- `$ans_size = 2`: (Now there are two subsets: `[]` and `[1]`)
+- For `i = 0`:
+   - `$cur = []`: Take the empty subset.
+   - `$cur[] = 2`: Append `2` to it, resulting in `[2]`.
+   - `$ans = [[], [1], [2]]`: Add `[2]` to `$ans`.
+- For `i = 1`:
+   - `$cur = [1]`: Take the subset `[1]`.
+   - `$cur[] = 2`: Append `2` to it, resulting in `[1, 2]`.
+   - `$ans = [[], [1], [2], [1, 2]]`: Add `[1, 2]` to `$ans`.
+
+**Final Result**:
+- The function returns `[[], [1], [2], [1, 2]]`, which are all possible subsets of `[1, 2]`.
+
+### Summary:
+
+This function uses a dynamic approach to generate all subsets of an array by iteratively adding each element of the array to existing subsets. The time complexity is \(O(2^n)\), where \(n\) is the number of elements in the input array, because each element can either be included or excluded from each subset, resulting in \(2^n\) possible subsets.
+
+**Contact Links**
+
+If you found this series helpful, please consider giving the **[repository](https://github.com/mah-shamim/leet-code-in-php)** a star on GitHub or sharing the post on your favorite social networks 😍. Your support would mean a lot to me!
+
+If you want more helpful content like this, feel free to follow me:
+
+- **[LinkedIn](https://www.linkedin.com/in/arifulhaque/)**
+- **[GitHub](https://github.com/mah-shamim)**