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.

很锻炼DP/recursive思路的一道题，个人感觉DP/recursive算是比较难写的题目了。这道题解法的巧妙之处在于巧用-1，并且使用临时存储，节省了很多开支。这道题同时也在Career Cup上面出现过

这道题我两次调试通过，第一次错是因为input{}, output false, expected true

我的算法只有一层递归(因为巧用-1的原因)，runs in O(N) time, andO(H) space

public class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        if (checkBalance(root) != -1) return true;
        else return false;
    }
    
    public int checkBalance(TreeNode root) {
        if (root == null) return 0;
        int leftHeight = checkBalance(root.left);
        int rightHeight = checkBalance(root.right);
        if (leftHeight == -1 || rightHeight == -1) return -1;
        else if (Math.abs(leftHeight - rightHeight) > 1) return -1;
        return Math.max(leftHeight, rightHeight) + 1;
    }
}