leetcode 110 Balanced Binary Tree(DFS)

Given a binary tree, determine if it is height-balanced.

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

题意：给一棵二叉树，判断它是否是平衡二叉树（即每个节点的左右子树深度之差不大于1）.

题解：dfs的简单应用。有一个技巧是：用返回-1来表示不行。

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int dfs(TreeNode *r){
        if(r==NULL){
            return 0;
        }
        int ll=dfs(r->left);
        int rr=dfs(r->right);
        if(rr==-1||ll==-1||abs(ll-rr)>1){
            return -1;
        }
        else{
            return max(ll,rr)+1;
        }
    }
    bool isBalanced(TreeNode* root) {
        if(dfs(root)==-1){
            return false;
        }
        else{
            return true;
        }
    }
};