【LeetCode 0104】【递归/分治】二叉树的最大深度

104. Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

A binary tree’s maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example 1:

**Input:** root = [3,9,20,null,null,15,7]
**Output:** 3

Example 2:

**Input:** root = [1,null,2]
**Output:** 2

Constraints:

• The number of nodes in the tree is in the range [0, 10^4].
• -100 <= Node.val <= 100

Idea

分治递归：
分别算出左子树最大深度 和 右子树最大深度
返回 1+左右子树的最大深度

JavaScript Solution

/**
 * 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) {
    
    if(!root){
        return 0
    }
    return Math.max(maxDepth(root.left),maxDepth(root.right)) + 1;
};