104.Maximum Depth of Binary Tree

problems:
Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Note: A leaf is a node with no children.

Example:

Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its depth = 3.
tips:
求二叉树的最大深度。对于每个根节点，求得它左子树和右子树的最大深度然后加1即可。
solution:

class Solution {
public:
    int maxDepth(TreeNode* root) {
        int i,j;
        if(root == NULL) return 0;
        i = maxDepth(root->left)+1;
        j = maxDepth(root->right)+1;
        if(i>=j) return i;
        else return j;
    }
};