[LeetCode從零單刷]Balanced Binary Tree

題目：

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.

解答：

一開始看題目懵比了。後來一想，就是問樹的左右子樹高度相差是否 > 1。如果大於 1，可以記錄數高為特殊高度（-1），逐層向上傳遞 -1。

重要的是重新再寫一個函數計算樹的高度：

/**
 * 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 treeDepth(TreeNode* root)
    {
        if(root == NULL)
            return 0;
        else
        {
            int ldepth  = treeDepth(root->left);
            int rdepth  = treeDepth(root->right);
            if(ldepth == -1 || rdepth == -1)    return -1;
            
            int diff    = fabs(ldepth - rdepth);
            if(diff > 1)    return -1;
            else
            {
                return ((ldepth > rdepth)?ldepth: rdepth) + 1;
            }
        }
    }
    
    bool isBalanced(TreeNode* root) {
        int ans = treeDepth(root);
        if(ans == -1)   return false;
        else            return true;
    }
};