1519. Number of Nodes in the Sub-Tree With the Same Label ⭐

Medium

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 <= 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 lowercase English letters.

解題

⭐不要做一棵樹，想成graph⭐

Runtime: 278 ms, faster than 100%

Memory Usage: 48.7 MB, less than 25%

func countSubTrees(n int, edges [][]int, labels string) []int {
    ans := make([]int, n)

    graph := make([][]int, n) // 紀錄各個節點的鄰邊

    for _, edge := range edges {
        u := edge[0]
        v := edge[1]
        graph[u] = append(graph[u], v)
        graph[v] = append(graph[v], u)
    }

    var dfs func(int, int) []int
    dfs = func(u int, parent int) []int {
        count := make([]int, 26)
        
        for _, v := range graph[u] { // 加總子節點的label
            if v == parent { continue } // 不會往上(parent)數
            childCount := dfs(v, u)

            for i:=0; i<26; i++ {
                count[i] += childCount[i] 
            }
        }

        count[labels[u] - 'a']++ // 把自己的label也加上去
        ans[u] =  count[labels[u] - 'a'] // 紀錄這個節點的答案

        return count
    }

    dfs(0, -1)

    return ans
}