Leetcode 814. Binary Tree Pruning

We are given the head node root of a binary tree, where additionally every node's value is either a 0 or a 1.

Return the same tree where every subtree (of the given tree) not containing a 1 has been removed.

(Recall that the subtree of a node X is X, plus every node that is a descendant of X.)

Example 1:
Input: [1,null,0,0,1]
Output: [1,null,0,null,1]
 
Explanation: 
Only the red nodes satisfy the property "every subtree not containing a 1".
The diagram on the right represents the answer.

Example 2:
Input: [1,0,1,0,0,0,1]
Output: [1,null,1,null,1]

Example 3:
Input: [1,1,0,1,1,0,1,0]
Output: [1,1,0,1,1,null,1]

Note:

• The binary tree will have at most 100 nodes.
• The value of each node will only be 0 or 1.

Code (Java):

/*
 * @lc app=leetcode id=814 lang=java
 *
 * [814] Binary Tree Pruning
 */
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode pruneTree(TreeNode root) {
        if (root == null) {
            return null;
        }

        ResultType ans = pruneTreeHelper(root);

        return ans.root;
    }

    private ResultType pruneTreeHelper(TreeNode root) {
        if (root == null) {
            return new ResultType(null, true);
        }

        ResultType left = pruneTreeHelper(root.left);
        ResultType right = pruneTreeHelper(root.right);

        if (left.isZeroTree) {
            root.left = null;
        }

        if (right.isZeroTree) {
            root.right = null;
        }

        boolean isZeroTree = left.isZeroTree && right.isZeroTree && root.val == 0;
        
        return new ResultType(root, isZeroTree);
    }
}

class ResultType {
    TreeNode root;
    boolean isZeroTree;

    public ResultType(TreeNode root, boolean isZeroTree) {
        this.root = root;
        this.isZeroTree = isZeroTree;
    }
}