68：Balanced Binary Tree

题目：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.

该题目应选择递归解法，但递归解法有两种版本，第二种版本要比第一种版本更好

递归版本一

// 迭代版本一
// 该版本缺点就是节点高度值会重复计算
class Solution {
public:
        bool isBalanced(TreeNode* root) {
                if (!root) return true;

                int diff = abs(height(root -> left) - height(root -> right))
                if (diff <= 1) return isBalanced(root -> left) && isBalanced(root -> right);
                else return false;
        }
private:
        int height(TreeNode* root) {
                if (!root) return 0;

                int h = max(height(p -> left), height(p -> right)) + 1;
                return h;
        }
};

递归版本二：

// 迭代版本 二
// 时间复杂度 O(n)，空间复杂度 O(log n)
class Solution {
public:
        bool isBalanced(TreeNode* root) {
                return balancedHeight(root) >= 0;
        }

        int balancedHeight(TreeNode* root) {
                if (!root) return 0;

                int left = balancedHeight(root -> left);
                int right = balancedHeight(root -> right);

                if (left < 0 || right < 0 || abs(left - right) > 1) return -1; // 剪枝

                return max(left, right) + 1;
        }
};