删除树节点

/*
 * Problem 1273: Delete Tree Nodes
 * Given a tree represented by an integer array nodes, where nodes[i] is the value of the ith node. 
 * The tree has n nodes with node 0 as the root. The array parent of length n, where parent[i] is the parent of the ith node.
 * Delete the subtrees of nodes with a value of 0 and return the number of remaining nodes.
 * 
 * Approach using Java Streams:
 * 1. Parse the parent array to construct the tree structure.
 * 2. Use a depth-first search (DFS) to traverse the tree using streams.
 * 3. Calculate the sum of subtree values and count nodes.
 * 4. If the sum of a subtree is zero, exclude it from the count.
 */

import java.util.*;
import java.util.stream.*;

public class Solution {
    public int deleteTreeNodes(int nodes, int[] parent, int[] value) {
        Map<Integer, List<Integer>> tree = IntStream.range(0, nodes).boxed()
            .collect(Collectors.groupingBy(i -> parent[i]));

        return dfs(0, tree, value)[1];
    }

    private int[] dfs(int node, Map<Integer, List<Integer>> tree, int[] value) {
        int sum = value[node];
        int count = 1;
        if (tree.containsKey(node)) {
            for (int child : tree.get(node)) {
                int[] result = dfs(child, tree, value);
                sum += result[0];
                count += result[1];
            }
        }
        if (sum == 0) {
            count = 0;
        }
        return new int[]{sum, count};
    }
}