110. 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 left and right subtrees of every node differ in height by no more than 1.

Example 1:

Input: root = [3,9,20,null,null,15,7]
Output: true

ex1

Example 2:

Input: root = [1,2,2,3,3,null,null,4,4]
Output: false

ex1

Example 3:

Input: root = []
Output: true

Constraints:

  • The number of nodes in the tree is in the range [0, 5000].
  • -104 <= Node.val <= 104
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        if (nums.length == 0) return null;
        return buildBST(nums, 0, nums.length - 1);
    }
    
    TreeNode buildBST(int[] nums, int lo, int hi) {
        if (hi < lo) {
            return null;
        }
        TreeNode node = new TreeNode();
        int mid = lo +  (hi  - lo) / 2;
        node.val = nums[mid];
        node.left = buildBST(nums, lo, mid - 1);
        node.right = buildBST(nums, mid + 1, hi);
        return node;
    }
}

Solution 2021-11-08

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    
    public boolean isBalanced(TreeNode root) {
        return getHeight(root) != -1;
    }
    
    int getHeight(TreeNode node) {
        if (node == null) return 0;
        
        int left = getHeight(node.left);
        int right = getHeight(node.right);
        
        if (left == -1 || right == -1) return -1;
        
        if (Math.abs(left - right) > 1) {
            return -1;
        }
        
        return Math.max(left, right) + 1;
    }
    
    public boolean isBalancedTopDown(TreeNode root) {
        if (root == null) return true;
        
        int left = longest(root.left);
        int right = longest(root.right);

        return Math.abs(left - right) <= 1 && isBalanced(root.left) && isBalanced(root.right);
    }
    
    int longest(TreeNode node) {
        if (node == null) return 0;
        return Math.max(longest(node.left), longest(node.right)) + 1;
    }
}

Solution 2021-11-20

class Solution {
    public boolean isBalanced(TreeNode root) {
        if (root == null) return true;
        
        int left = height(root.left);
        int right = height(root.right);
        
        // BOTTOM-UP
        return isBalanced(root.left) && isBalanced(root.right) && Math.abs(left - right) <= 1;

        // TOP-DOWN
        // return Math.abs(left - right) <= 1 && isBalanced(root.left) && isBalanced(root.right);
    }
    
    int height(TreeNode node) {
        if (node == null) return 0;
        
        return Math.max(height(node.left), height(node.right)) + 1;
    }
}