104 Maximum Depth of Binary Tree

方法一：深度优先遍历
class Solution {
public:
    int maxDepth(TreeNode *root) {
        if(root==NULL)
            return 0;
        if(root->left==NULL&&root->right==NULL)
            return 1;
        int mx=0;
        if(root->left)
            mx=maxDepth(root->left)+1;
        if(root->right)
            mx=max(maxDepth(root->right)+1, mx);
        return mx;
    }
};