leetcode 刷题之路 86 Balanced Binary Tree

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.

判断一个二叉树是不是平衡二叉树，如果二叉树的每个节点的左右子树的深度相差不超过1，此二叉树为平衡二叉树。

思路，二叉树root为平衡二叉树的条件是，它的左右子树为平衡二叉树，左右子树的深度差不超过1，因此首先递归判断左右子树是否平衡，同时在递归过程中通过引用传参的方式传递回左右子树的深度，然后再根据左右子树的深度做出整体判断。

AC code：

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    bool isBalanced(TreeNode *root) 
    {
        int depth;
        return helper(root,depth);
    }
    bool helper(TreeNode *root,int &depth)
    {
        if(root==NULL)
        {
            depth=0;
            return true;
        }
        int leftDepth,rightDepth;
        bool b1,b2;
        if(!helper(root->left,leftDepth))
            return false;
        if(!helper(root->right,rightDepth))
            return false;
        if(abs(rightDepth-leftDepth)>1)
            return false;
        else
        {
            depth=rightDepth>leftDepth?rightDepth+1:leftDepth+1;
            return true;
        }
    }
    
};