104. Maximum Depth of Binary Tree LeetCode

题意：求二叉树的最大深度。
题解：递归求，从根节点出发的最大深度等于从左儿子出发的最大深度+1或者从右儿子出发的最大深度加1.

class Solution {public:    int maxDepth(TreeNode* root) {        if(root == NULL) return 0;        if(root->left == NULL && root->right == NULL) return 1;        int l = 0,r = 0;        if(root->left != NULL) l = maxDepth(root->left);        if(root->right != NULL) r = maxDepth(root->right);        return max(l,r) + 1;    }};