【leetcode】110. Balanced Binary Tree【java】

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.

解题思路来自 https://discuss.leetcode.com/topic/7798/the-bottom-up-o-n-solution-would-be-better

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
 
 //O(NlogN)
public class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null){
            return true;
        }
        return Math.abs(Height(root.left) - Height(root.right)) <= 1 && isBalanced(root.left) && isBalanced(root.right);
    }
    public int Height(TreeNode root) {
        if (root == null){
            return 0;
        }
        return Math.max(Height(root.left), Height(root.right)) + 1;
    }
}
//O(N)  这个时间复杂度更低一些
public class Solution {
   public boolean isBalanced(TreeNode root) {
        return maxDepth(root) != -1;
    }
    
    public int maxDepth(TreeNode root) {
        if (root == null){
            return 0;
        }
        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        if (left == -1 || right == -1 || Math.abs(left  - right) > 1) {
            return -1;
        }
        return Math.max(left, right) + 1;
    }
}