1519.CountSubTrees.cs

// 1519. Number of Nodes in the Sub-Tree With the Same Label
// You are given a tree (i.e. a connected, undirected graph that has no cycles) consisting of n nodes numbered from 0 to n - 1 and exactly n - 1 edges. The root of the tree is the node 0, and each node of the tree has a label which is a lower-case character given in the string labels (i.e. The node with the number i has the label labels[i]).
// The edges array is given on the form edges[i] = [ai, bi], which means there is an edge between nodes ai and bi in the tree.
// Return an array of size n where ans[i] is the number of nodes in the subtree of the ith node which have the same label as node i.
// A subtree of a tree T is the tree consisting of a node in T and all of its descendant nodes.
// Example 1:
// Input: n = 7, edges = [[0,1],[0,2],[1,4],[1,5],[2,3],[2,6]], labels = "abaedcd"
// Output: [2,1,1,1,1,1,1]
// Explanation: Node 0 has label 'a' and its sub-tree has node 2 with label 'a' as well, thus the answer is 2. Notice that any node is part of its sub-tree.
// Node 1 has a label 'b'. The sub-tree of node 1 contains nodes 1,4 and 5, as nodes 4 and 5 have different labels than node 1, the answer is just 1 (the node itself).
// Example 2:
// Input: n = 4, edges = [[0,1],[1,2],[0,3]], labels = "bbbb"
// Output: [4,2,1,1]
// Explanation: The sub-tree of node 2 contains only node 2, so the answer is 1.
// The sub-tree of node 3 contains only node 3, so the answer is 1.
// The sub-tree of node 1 contains nodes 1 and 2, both have label 'b', thus the answer is 2.
// The sub-tree of node 0 contains nodes 0, 1, 2 and 3, all with label 'b', thus the answer is 4.
// Example 3:
// Input: n = 5, edges = [[0,1],[0,2],[1,3],[0,4]], labels = "aabab"
// Output: [3,2,1,1,1]
// Constraints:
// 1 <= n <= 105
// edges.length == n - 1
// edges[i].length == 2
// 0 <= ai, bi < n
// ai != bi
// labels.length == n
// labels is consisting of only of lowercase English letters.

public class Solution {
    public int[] CountSubTrees(int n, int[][] edges, string labels) {
       int[] res = new int[n];
        List<List<int>> adj = new List<List<int>>();
        for (int i = 0; i < n; i++)
            adj.Add(new List<int>());
        foreach (var edge in edges)
        {
            adj[edge[0]].Add(edge[1]);
            adj[edge[1]].Add(edge[0]);
        }
        DFS(0, -1, adj, labels, res);
        return res;
    }
    int[] DFS(int idx, int parent, List<List<int>> adj,
                string labels, int[] res)
    {
        int[] freq = new int[26];
        freq[labels[idx] - 'a'] = 1;
        foreach (var nbr in adj[idx])
        {
            if (nbr != parent)
            {
                int[] temp = DFS(nbr, idx, adj, labels, res);
                for(int i = 0; i < 26; i++)
                    freq[i] += temp[i];
            }
        }
        res[idx] = freq[labels[idx] - 'a'];
        return freq;
    }
}