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 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.

求左右子树的高度，判断左右子树高度差是否不超过1.

树的深度是从根节点开始(其深度为1)自顶向下逐层累加的,而高度是从叶节点开始(其高度为1)自底向上逐层累加的。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
private:
    bool balance = true;
    int height(TreeNode *root)
    {
        if(!balance)
            return -1;
        if(root==NULL)
            return 0;
        else
        {
            int left_height = height(root->left)+1;
            int right_height = height(root->right)+1;
            if( abs(left_height-right_height) > 1)
                balance = false;
            else
                return left_height>right_height ? left_height : right_height;
        }
    }

public:
    bool isBalanced(TreeNode *root) {
        height(root);
        return balance;
    }
};