I have heard people say it is easy but it took me some time to get this… 🙁

public class BalancedTree {
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
public boolean isBalanced(TreeNode root) {
if (root == null) {
return true;
} else {
int[] lh = _isBalanced(root);
return lh == null ? false : true;
}
}
private int[] _isBalanced(TreeNode root) {
int[] def = new int[2];
def[0] = 0;
def[1] = 0;
if (root.left != null) {
int[] tmp = _isBalanced(root.left);
if (tmp != null) {
def[0] = Math.max(tmp[0], tmp[1]) + 1;
} else {
return null;
}
}
if (root.right != null) {
int[] tmp = _isBalanced(root.right);
if (tmp != null) {
def[1] = Math.max(tmp[0], tmp[1]) + 1;
} else {
return null;
}
}
if (def == null || Math.abs(def[0] - def[1]) > 1) {
return null;
} else {
return def;
}
}
}

*Related*