2020.11.9

14.最长公共前缀.java

@@ -7,7 +7,6 @@
 // @lc code=start
 class Solution {
     public String longestCommonPrefix(String[] strs) {
-       // StringBuilder ans = new StringBuilder();
         if(strs.length == 1){
             return strs[0];
         }

15.三数之和.cpp

@@ -0,0 +1,15 @@
+/*
+ * @lc app=leetcode.cn id=15 lang=cpp
+ *
+ * [15] 三数之和
+ */
+
+// @lc code=start
+class Solution {
+public:
+    vector<vector<int>> threeSum(vector<int>& nums) {
+
+    }
+};
+// @lc code=end
+

16.最接近的三数之和.java

@@ -0,0 +1,40 @@
+import java.util.Arrays;
+/*
+ * @lc app=leetcode.cn id=16 lang=java
+ *
+ * [16] 最接近的三数之和
+ */
+
+// @lc code=start
+class Solution {
+    public int threeSumClosest(int[] nums, int target) {
+        int ans = nums[0] + nums[1] + nums[2];
+        int index = 0;
+        int t = Integer.MAX_VALUE;
+        Arrays.sort(nums);
+        int length  = nums.length;
+        if(nums.length == 3){
+            return nums[0] + nums[1] + nums[2];
+        }
+        if( nums[0] + nums[1] + nums[2] > target){
+            return  nums[0] + nums[1] + nums[2];
+        }
+        if(nums[length - 1] + nums[length - 2] + nums[length- 3] < target){
+            return nums[length - 1] + nums[length - 2] + nums[length- 3];
+        }  
+        for(int i = 0;i<nums.length - 2;i++){
+            for(int j = i+ 1;j<nums.length - 1;j++){
+                for(int k = j + 1;k<nums.length;k++){
+                    index = Math.abs(nums[i] + nums[j] + nums[k] - target);
+                    if(index < t){
+                        t = index;
+                        ans = nums[i] + nums[j] + nums[k];
+                    }
+                }
+            }
+        }
+        return ans;
+    }
+}
+// @lc code=end
+

17.电话号码的字母组合.cpp

@@ -0,0 +1,37 @@
+/*
+ * @lc app=leetcode.cn id=17 lang=cpp
+ *
+ * [17] 电话号码的字母组合
+ */
+
+// @lc code=start
+class Solution
+{
+public:
+    string tmp;
+    vector<string> res;
+    vector<string> board = {"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
+    void DFS(int pos, string digits)
+    {
+        if (pos == digits.size())
+        {
+            res.push_back(tmp);
+            return;
+        }
+        int num = digits[pos] - '0';
+        for (int i = 0; i < board[num].size(); i++)
+        {
+            tmp.push_back(board[num][i]);
+            DFS(pos + 1, digits);
+            tmp.pop_back();
+        }
+    }
+    vector<string> letterCombinations(string digits)
+    {
+        if (digits.size() == 0)
+            return res;
+        DFS(0, digits);
+        return res;
+    }
+};
+// @lc code=end

17.电话号码的字母组合.java

@@ -0,0 +1,14 @@
+/*
+ * @lc app=leetcode.cn id=17 lang=java
+ *
+ * [17] 电话号码的字母组合
+ */
+
+// @lc code=start
+class Solution {
+    public List<String> letterCombinations(String digits) {
+
+    }
+}
+// @lc code=end
+

18.四数之和.cpp

@@ -0,0 +1,15 @@
+/*
+ * @lc app=leetcode.cn id=18 lang=cpp
+ *
+ * [18] 四数之和
+ */
+
+// @lc code=start
+class Solution {
+public:
+    vector<vector<int>> fourSum(vector<int>& nums, int target) {
+
+    }
+};
+// @lc code=end
+

18.四数之和.java

@@ -0,0 +1,53 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.List;
+
+/*
+ * @lc app=leetcode.cn id=18 lang=java
+ *
+ * [18] 四数之和
+ */
+
+// @lc code=start
+class Solution {
+    List<List<Integer>> res;
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        res = new ArrayList<>();
+        if(nums.length == 0){
+            return res;
+        }
+        Arrays.sort(nums);
+        DFS(target, nums, 0, 0, 0, new ArrayList<Integer>());
+        return res;
+    }
+
+    void DFS(int target, int[] nums,int cur, int size, int sum, List<Integer> list){
+        if(list.size() == 4){
+            if(sum == target){
+                res.add(new ArrayList<>(list));
+            }
+            return;
+        }
+
+        Set<Integer> set = new HashSet<>();
+        for(int i = cur;i<nums.length;i++){
+            //去重
+            if(set.contains(nums[i])) continue;
+            //剪枝： 当剩余元素个数不满足所需个数时返回
+            if(nums.length - i < (4 - size)) return;
+            //剪枝： 当剩余所需元素都用后续最小元素填充时还是超过了target返回
+            if(i < nums.length - 1 && sum + nums[i] + (3 - size) * nums[i + 1] > target) return;
+            //剪枝： 当剩余所需元素都用后续最大元素填充时还是小于target，则返回
+            if(i < nums.length - 1 && sum + nums[i] + (3 - size) * nums[nums.length - 1] < target) continue;
+
+            //回溯
+            set.add(nums[i]);
+            list.add(nums[i]);
+            DFS(target, nums, i + 1, size + 1, sum + nums[i], list);
+            list.remove(list.size() - 1);
+        }
+    }
+}
+// @lc code=end
+

19.删除链表的倒数第n个节点.java

@@ -0,0 +1,51 @@
+/*
+ * @lc app=leetcode.cn id=19 lang=java
+ *
+ * [19] 删除链表的倒数第N个节点
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    public ListNode removeNthFromEnd(ListNode head, int n) {
+        ListNode index = head;
+        int sum = 0;
+        while(index.next != null){
+            sum++;
+            index = index.next;
+        }
+        sum++;//5
+        if(sum == n){
+            head = head.next;
+            return head;
+        }
+
+        int i = 0;
+        ListNode left = head;
+        index = head;
+        while(i < sum - n){
+            index = index.next;
+            if(i > 0){
+                left = left.next;
+            }
+            i++;
+        }
+        if(index.next == null){
+            left.next = null;
+            return head;
+        }
+        left.next = index.next;
+        return head;
+    }
+}
+// @lc code=end
+

20.有效的括号.java

@@ -0,0 +1,23 @@
+import java.util.LinkedList;
+
+/*
+ * @lc app=leetcode.cn id=20 lang=java
+ *
+ * [20] 有效的括号
+ */
+
+// @lc code=start
+class Solution {
+    public boolean isValid(String s) {
+        LinkedList<Character> stack = new LinkedList<>();
+        for(char c : s.toCharArray()){
+            if(c == '[') stack.push(']');
+            else if(c == '{') stack.push('}');
+            else if(c == '(') stack.push(')');
+            else if(stack.isEmpty() || c != stack.pop()) return false;
+        }
+        return stack.isEmpty();
+    }
+}
+// @lc code=end
+

21.合并两个有序链表.java

@@ -0,0 +1,67 @@
+import java.util.ArrayList;
+import java.util.Arrays;
+
+import javax.swing.ListModel;
+
+/*
+ * @lc app=leetcode.cn id=21 lang=java
+ *
+ * [21] 合并两个有序链表
+ */
+
+// @lc code=start
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode() {}
+ *     ListNode(int val) { this.val = val; }
+ *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
+ * }
+ */
+class Solution {
+    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
+        ListNode res = new ListNode();
+        if(l1 == null && l2 != null){
+            return l2;
+        }else if(l1 != null && l2 == null){
+            return l1;
+        }
+        else if(l1 == null && l2 == null){
+            return null;
+        }
+        ListNode head = res;
+        ArrayList<Integer> list = new ArrayList<>();
+        list.add(l1.val);
+        list.add(l2.val);
+        int i = 2;
+        while(l1.next != null){
+            l1 = l1.next;
+            list.add(l1.val);
+            i++;
+        }
+        while(l2.next != null){
+            l2 = l2.next;
+            list.add(l2.val);
+            i++;
+        }
+        list.sort(Comparator.naturalOrder());
+        int length = i;
+        i = 0;
+        while(i < length){
+            ListNode index = new ListNode();
+            res.val = list.get(i);
+            if(i < length - 1){
+                res.next = index;
+                res = res.next;
+                i++;
+            }
+            else{
+                i++;
+            }
+        }
+        return head;
+    }
+}
+// @lc code=end

22.括号生成.java

@@ -0,0 +1,38 @@
+import java.util.ArrayList;
+
+/*
+ * @lc app=leetcode.cn id=22 lang=java
+ *
+ * [22] 括号生成
+ */
+
+// @lc code=start
+class Solution {
+    List<String> res = new ArrayList<>();
+    public List<String> generateParenthesis(int n) {
+        if(n <= 0){
+            return res;
+        }
+        getParenthesis("",n,n);
+        return res;
+    }
+
+    private void getParenthesis(String str, int left, int right){
+        if(left == 0 && right == 0){
+            res.add(str);
+            return;
+        }
+
+        if(left  == right){
+            getParenthesis(str + '(', left - 1, right);
+        }
+        else if(left < right){
+            if(left > 0){
+                getParenthesis(str + '(', left - 1, right);
+            }
+            getParenthesis(str + ')', left, right - 1);
+        }
+    }
+}
+// @lc code=end
+