[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;
    }
};