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.

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).

Solution

/**
 * @param {number} n
 * @param {number[][]} edges
 * @param {string} labels
 * @return {number[]}
 */
var countSubTrees = function (n, edges, labels) {
  // build graph
  const graph = new Map();
  // init graph
  for (let i = 0; i < n; i++) {
    graph.set(i, []);
  }
  // add edges
  for (let [a, b] of edges) {
    graph.get(a).push(b);
    graph.get(b).push(a);
  }
  const ans = new Array(n).fill(0);
  const seen = new Set();
  const dfs = (node) => {
    const count = new Array(26).fill(0);
    seen.add(node);
    for (let nei of graph.get(node)) {
      if (!seen.has(nei)) {
        const sub = dfs(nei);
        for (let i = 0; i < 26; i++) {
          count[i] += sub[i];
        }
      }
    }
    count[labels.charCodeAt(node) - 97]++;
    ans[node] = count[labels.charCodeAt(node) - 97];
    return count;
  };
  dfs(0);
  return ans;
};