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.

Version1: traverse each node only one time, the complexity is O(N)

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

Version2: traverse each node repeatedly and the complexity is O(N2)

public class Solution {    public int levels(TreeNode root) {        if(root==null) return 0;                int lheight = levels(root.left) + 1;        int rheight = levels(root.right) + 1;        return Math.max(lheight, rheight);    }        public boolean isBalanced(TreeNode root) {        if(root==null) return true;                int lheight = levels(root.left);        int rheight = levels(root.right);        if(Math.abs(lheight-rheight)>1) return false;        return isBalanced(root.left) && isBalanced(root.right);    }}