_110.java

package com.fishercoder.solutions;

import com.fishercoder.common.classes.TreeNode;

/**
 * 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.*/

public class _110 {
    
    class Solution_1 {
        //recursively get the height of each subtree of each node, compare their difference, if greater than 1, then return false
        //although this is working, but it's not efficient, since it repeatedly computes the heights of each node every time
        //Its time complexity is O(n^2).
        public boolean isBalanced(TreeNode root) {
            if(root == null) return true;
            if(Math.abs(getH(root.left) - getH(root.right)) > 1) return false;
            else return isBalanced(root.left) && isBalanced(root.right);
        }

        private int getH(TreeNode root) {
            if(root == null) return 0;//base case
            int leftH = getH(root.left);
            int rightH = getH(root.right);
            return Math.max(leftH, rightH)+1;
        }
    }
    
    class Solution_2 {

        public boolean isBalanced(TreeNode root) {
            return getH(root) != -1;
        }

        private int getH(TreeNode root) {
            if (root == null) return 0;
            int leftH = getH(root.left);
            if (leftH == -1) return -1;
            int rightH = getH(root.right);
            if (rightH == -1) return -1;
            if (Math.abs(leftH - rightH) > 1) return -1;
            return Math.max(leftH, rightH) + 1;
        }
    }

}