111.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number}
 */

// BFS with inner loop to iterate over level
var minDepth = function(root) {
    if (!root) {
        return 0
    }
    
    const queue = [root]
    let depth = 1
    while(queue.length) {
        const len = queue.length;
        for(let i = 0; i < len; i++) {
            const node = queue.shift()
            if (!node.left && !node.right) {
                return depth
            }
            if (node.left) {
                queue.push(node.left)
            }
            if (node.right) {
                queue.push(node.right)
            }
        }
        depth++
    }
};

// DFS with temporary array for level 
var minDepth = function(root) {
    if (!root) {
        return 0
    }
    
    let queue = [root]
    let temp = []
    let depth = 1
    while(queue.length || temp.length) {
        const node = queue.pop()
        if (!node.left && !node.right) {
            return depth
        }
        if (node.left) {
            temp.push(node.left)
        }
        if (node.right) {
            temp.push(node.right)
        }

        if (!queue.length) {
            depth++
            queue = temp
            temp = []
        }
    }
};