Codeforces 813 C The Tag Game

题目地址：http://codeforces.com/contest/813/problem/C
题意：一个改版的Tag Game，他就是告诉你一个树状图，然后告诉你点的编号，代表Alice和Bob，每个人可以走到相邻的点或者在原地等，希望Alice尽可能走，Bob希望走较少的步，先是Alice走，求出多少步以后Alice和Bob可以相遇。
思路：从Alice和Bob的点分别出发求最短路，再遍历len数组，找到lena[i]>lenb[i]的所有点，找里面lena[i]的最大值，结果就是lena[i]*2。

#include <iostream>#include <cstring>#include <string>#include <queue>#include <vector>#include <map>#include <algorithm>#define N 200010#define LL long long #define inf 0x3f3f3f3fusing namespace std;vector<int> mapp[N];bool vis[N];int len[2][N];int n;void init() {    for (int i = 0; i <= n; i++) {        mapp[i].clear();    }    memset(len, inf, sizeof(len));}void spfa(int ans,int cnt) {    memset(vis, false, sizeof(vis));    queue<int> q;    q.push(ans);    vis[ans] = true;    len[cnt][ans] = 0;    while (!q.empty()) {        int u = q.front();        q.pop();        for (int i = 0; i < mapp[u].size(); i++) {            int v = mapp[u][i];            if (len[cnt][v] > len[cnt][u] + 1) {                len[cnt][v] = len[cnt][u] + 1;                if (!vis[v]) {                    vis[v] = true;                    q.push(v);                }            }        }        vis[u] = false;    }}int main() {    int a, b;    int x;    int ans;    cin.sync_with_stdio(false);    while (cin >> n >> x) {        init();        for (int i = 1; i < n; i++) {            cin >> a >> b;            mapp[a].push_back(b);            mapp[b].push_back(a);        }        spfa(1, 0);        spfa(x, 1);        ans = 0;        for (int i = 1; i <= n; i++) {            if (len[0][i] > len[1][i]) {                ans = max(ans, len[0][i]);            }        }        cout << ans * 2 << endl;    }    return 0;}