思路

深度优先遍历中记录节点的层级

代码

先深度优先遍历一下

var maxDepth = function(root) {
    const dfs = (n) => {
        if(n){
            log(n.val)
            dfs(n.left)
            dfs(n.right)
        }

    }
    dfs(root)
};

加上l

每调用一下dfs就l+1

var maxDepth = function(root) {
    let res = 0
    const dfs = (n,l) => {
        if(n){
            if(!n.left && !n.right){
                res = Math.max(res,l)
            }
            dfs(n.left,l + 1)
            dfs(n.right,l + 1)
        }

    }
    dfs(root,1)
    return res
};

非递归

/**
 * 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) {
    let res = 0
    const dfs = (root) => {
        if(root){
           const stack = [[root,1]]
           while(stack.length){
               const [n,l] = stack.pop()
               if(!n.left && !n.right){
                   res = Math.max(res,l)
               }
               if(n.left) stack.push([n.left,l+1])
               if(n.right) stack.push([n.right,l+1])
           }
        }

    }
    dfs(root,1)
    return res
};

复杂度

时间:O(n)

空间:O(n),函数调用函数的情况会自动形成一个栈