Even Tree 小议

原题链接：https://www.hackerrank.com/challenges/even-tree/problem

思路：把树还原出来，对每个结点，计算以该结点为根的子树结点数。子树结点数为偶数的结点数量就是所求森林中树木的数量。结点数量减去一就是所求结果。

 #include <cmath>
 #include <cstdio>
 #include <vector>
 #include <iostream>
 #include <algorithm>
 using namespace std;

 struct node{
     int index, num_subtree;
     vector<node*> children;
     node(int i=,int n=):
         index(i),num_subtree(n), children(vector<node*>()){}
 };

 node* construct_tree(int parent, int index, const vector<vector<int>>& adjLsit);
 void count_even(node* root, int& cnt);

 int main() {
     /* Enter your code here. Read input from STDIN. Print output to STDOUT */
     int N, M; cin >> N >> M;
     vector<vector<int>> adjList(N + );
     for(int i = ; i < M; i++){
         int n1, n2; cin >> n1 >> n2;
         adjList[n1].push_back(n2);
         adjList[n2].push_back(n1);
     }
     node* root = construct_tree(, , adjList);
     int cnt = ;
     count_even(root, cnt);
     cout << cnt -  << endl;
     return ;
 }

 node* construct_tree(int parent, int index, const vector<vector<int>>& adjList){
     node* tmp = new node(index);
     tmp->num_subtree = ;
     for(int i = ; i < adjList[index].size(); i++){
         if(adjList[index][i] != parent){
             tmp->children.push_back(construct_tree(index, adjList[index][i], adjList));
             tmp->num_subtree += tmp->children.back()->num_subtree;
         }
     }
     return tmp;
 }

 void count_even(node* root, int& cnt){
     for(auto vit: root->children){
         count_even(vit, cnt);
     }
     if(root->num_subtree%==){
         cnt++;
     }
 }