[LeetCode] Problem 863 - All Nodes Distance K in Binary Tree

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We are given a binary tree (with root node root), a target node, and an integer value K.

Return a list of the values of all nodes that have a distance K from the target node. The answer can be returned in any order.

Example

Input: root = [3,5,1,6,2,0,8,null,null,7,4], target = 5, K = 2

Output: [7,4,1]

Explanation:
The nodes that are a distance 2 from the target node (with value 5)
have values 7, 4, and 1.

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Note that the inputs “root” and “target” are actually TreeNodes.
The descriptions of the inputs above are just serializations of these objects.

Note:

  1. The given tree is non-empty.
  2. Each node in the tree has unique values 0 <= node.val <= 500.
  3. The target node is a node in the tree.
  4. 0 <= K <= 1000.

Code

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public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}
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public List<Integer> distanceK(TreeNode root, TreeNode target, int K) {
    List<Integer> result = new ArrayList<>();
    helper(result, root, target, K);
    return result;
}

private int helper(List<Integer> result, TreeNode root, TreeNode target, int K) {
    if (root == null)
        return -1;

    if (root == target) {
        collect(result, root, K);
        return 0;
    }

    int left = helper(result, root.left, target, K);
    int right = helper(result, root.right, target, K);

    if (left >= 0) {
        if (left + 1 == K)
            result.add(root.val);

        collect(result, root.right, K - left - 2);
        return left + 1;
    }
    else if (right >= 0) {
        if (right + 1 == K)
            result.add(root.val);

        collect(result, root.left, K - right - 2);
        return right + 1;
    }
    else
        return -1;
}

private void collect(List<Integer> result, TreeNode root, int K) {
    if (root == null || K < 0)
        return;

    if (K == 0) {
        result.add(root.val);
        return;
    }

    collect(result, root.left, K - 1);
    collect(result, root.right, K - 1);
}