LeetCode 104. Maximum Depth of Binary Tree

用递归的办法

思路1，从下往上累加

    int maxDepth(TreeNode* root) {
        if(!root)
            return 0;
        return max(maxDepth(root->left), maxDepth(root->right))+1;
    }

思路2，从上往下累加

    void helper(TreeNode* p, int dep, int& depth){
        if(!p)
            return;
        
        depth=max(depth,dep+1);        
        helper(p->left, dep+1, depth);
        helper(p->right, dep+1, depth);
    }
    
    int maxDepth(TreeNode* root) {        
        int depth=0;
        helper(root, 0,depth);
        return depth;
    }