BalancedBinaryTree.java

package easy;

import classes.TreeNode;

public class BalancedBinaryTree {
    
    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) {
            if (root == null)
                return true;
            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;
        }
    }

}