Number of Nodes in the Sub-Tree With the Same Label

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]

Example 4:

Input: n = 6, edges = [[0,1],[0,2],[1,3],[3,4],[4,5]], labels = "cbabaa"
Output: [1,2,1,1,2,1]

Example 5:

Input: n = 7, edges = [[0,1],[1,2],[2,3],[3,4],[4,5],[5,6]], labels = "aaabaaa"
Output: [6,5,4,1,3,2,1]

Constraints:

• 1 <= n <= 10^5

• edges.length == n - 1

• edges[i].length == 2

• 0 <= ai, bi < n

• ai != bi

• labels.length == n

• labels is consisting of only of lower-case English letters.

class Solution {
    static class tree {
        int t;
        char label;
        ArrayList<tree> list;

        tree(char c, int t) {
            label = c;
            list = new ArrayList<>();
            this.t = t;
        }
    }

    Map<Character, Integer> map;
    int[] ans;

    public int[] countSubTrees(int n, int[][] edges, String labels) {
        map = new HashMap();
        ans = new int[n];
        // Creating tree
        Map<Integer, tree> map1 = new HashMap<>();
        for (int[] edge : edges) {
            int first = 0;
            if (!map1.containsKey(edge[0]))
                map1.put(edge[0], new tree(labels.charAt(edge[0]), edge[0]));
            else
                first = 1;
            if (!map1.containsKey(edge[1]))
                map1.put(edge[1], new tree(labels.charAt(edge[1]), edge[1]));
            else
                first = 2;
            if (first == 1 || edge[0] == 0)
                map1.get(edge[0]).list.add(map1.get(edge[1]));
            else
                map1.get(edge[1]).list.add(map1.get(edge[0]));
        }
        // DFS in post-Order
        helper(map1.get(0));
        return ans;
    }

    public void helper(tree node) {
        if (node == null)
            return;
        int prev = map.getOrDefault(node.label, 0);
        for (tree child : node.list)
            helper(child);
        map.put(node.label, map.getOrDefault(node.label, 0) + 1);
        ans[node.t] = map.get(node.label) - prev;
    }
}