110. Balanced Binary Tree

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Problem

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.

Example 1:

Given the following tree [3,9,20,null,null,15,7]:

    3
   / \
  9  20
    /  \
   15   7

Return true.

Example 2:

Given the following tree [1,2,2,3,3,null,null,4,4]:

       1
      / \
     2   2
    / \
   3   3
  / \
 4   4

Return false.

Code

class Solution {
    public boolean isBalanced(TreeNode root) {
        if(root == null) return true;

        int leftHeight = helper(root.left);
        int rightHeight = helper(root.right);

        if(Math.abs(leftHeight - rightHeight) > 1) return false;

        return isBalanced(root.left) && isBalanced(root.right);
    }

    private int helper(TreeNode root){
        if(root == null) return 0;

        return 1 + Math.max(helper(root.left), helper(root.right));
    }
}