[LeetCode] 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.

解题思路：

递归分别求左子树和右子树的高度，然后比较，如果他们相差小于1，则返回高度，否则返回-1。最后root结点若返回-1则false，否则true。

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution {

public:

    int depth(TreeNode *node)

    {

        if(node == NULL) return 0;

        int ld = 0, rd = 0;

        if(node -> left != NULL) ld = depth(node -> left);

        if(node -> right != NULL) rd = depth(node -> right);

        if(ld == -1 || rd == -1) return -1;

        if(abs(ld - rd) <= 1) return max(ld, rd) + 1;

        else return -1;

    }

    bool isBalanced(TreeNode *root) {

        // IMPORTANT: Please reset any member data you declared, as

        // the same Solution instance will be reused for each test case.

        if(depth(root) == -1) return false;

        else return true;

    }

};