1. 总是记住树遍历的套路
/*
struct TreeNode {
    int val;
    struct TreeNode *left;
    struct TreeNode *right;
    TreeNode(int x) :
            val(x), left(NULL), right(NULL) {
    }
};*/
class Solution {
public:
    int TreeDepth(TreeNode* pRoot) {
        if(pRoot==NULL) return 0;

        int l = TreeDepth(pRoot->left);
        int r = TreeDepth(pRoot->right);

        return max(l,r) +1 ;

    }
};