二叉树的最大深度

给定一个二叉树，找出其最大深度。

二叉树的深度为根节点到最远叶子节点的最长路径上的节点数。

说明: 叶子节点是指没有子节点的节点。

示例：给定二叉树 [3,9,20,null,null,15,7]，

返回它的最大深度 3 。

解法

层序遍历法，遍历完一层节点，则深度+1 这样，就可以得到最大深度。

/**
 * 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}
 */
var maxDepth = function(root) {
    const stack = [];
    const childStack = [];
    root !== null && stack.push(root);
    let deepNum = 0;
 
    while(childStack.length > 0 || stack.length > 0){
        if(stack.length > 0){
            deepNum++;
        }
        while(stack.length > 0){
            let current = stack.shift();
            current.left !== null && childStack.push(current.left);
            current.right !== null && childStack.push(current.right);
        }
        if(childStack.length > 0){
            deepNum++;
        }
        while(childStack.length > 0){
            let current = childStack.shift();
            current.left !== null && stack.push(current.left);
            current.right !== null && stack.push(current.right);
        }
    }
 
    return deepNum;
};

优化版：

/**
 * 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}
 */
var maxDepth = function(root) {
    const stack = [];
    if(root == null){
        return 0;
    }
    let deepNum = 0;
    stack.push(root);
 
    while(stack.length > 0){
        let size = stack.length;
        while(size > 0){
            const current = stack.shift();
            current.left !== null && stack.push(current.left);
            current.right !== null && stack.push(current.right);
            size--;
        }
        deepNum++;
    }
    return deepNum;
};

二叉树的最大深度

解法​

优化版：​

解法

优化版：