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. 

 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 * int val;
 * TreeNode left;
 * TreeNode right;
 * TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public boolean isBalanced = true;
    public boolean isBalanced(TreeNode root) {
        int tree_height = height (root);
        return isBalanced;
    }
    
    /*   A helper function that:
     *  (1) keep track of leftHeight/rightHeight 
     *  (2) update int isBalanced                
     */
    public int height (TreeNode root) {
        if (root == null) {
            return 0; 
        }
        int leftHeight = height(root.left);
        int rightHeight = height(root.right);
        if (leftHeight == -1 || rightHeight == -1) {
            return -1; 
        }
        
        if (Math.abs(leftHeight - rightHeight) > 1) {
            isBalanced = false; 
            return -1;
        }
        return Math.max(height(root.left), height(root.right)) + 1;
    }
}

Solution 2: a faster method (ref: 九章算法)

public class Solution {
    public boolean isBalanced(TreeNode root) {
        return maxDepth(root) != -1;
    }

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

        int left = maxDepth(root.left);
        int right = maxDepth(root.right);
        if (left == -1 || right == -1 || Math.abs(left-right) > 1) {
            return -1;
        }
        return Math.max(left, right) + 1;
    }
}
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