Codeforces Round #174 (Div. 1) B. Cow Program(dp + 记忆化)

题目链接：http://codeforces.com/contest/283/problem/B

思路： dp[now][flag]表示现在在位置now，flag表示是接下来要做的步骤，然后根据题意记忆化搜索记忆，vis数组标记那些已经访问过的状态。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#define REP(i, a, b) for (int i = (a); i < (b); ++i)
#define FOR(i, a, b) for (int i = (a); i <= (b); ++i)
using namespace std;

const int MAX_N = (200000 + 100);
long long dp[MAX_N][2];
int vis[MAX_N][2];
int N, a[MAX_N];

long long dfs(int now, int dir)
{
    if (now <= 0 || now > N) return 0;
    if (~dp[now][dir]) return dp[now][dir];
    if (vis[now][dir]) return -1;
    vis[now][dir] = 1;
    long long res = dfs(dir ? now + a[now] : now - a[now], dir ^ 1);
    return dp[now][dir] = (res < 0 ? -1 : res + a[now]);
}

int main()
{
    while (cin >> N) {
        FOR(i, 2, N) cin >> a[i];
        memset(dp, -1, sizeof(dp));
        memset(vis, 0, sizeof(vis));
        FOR(i, 1, N - 1) {
            long long ans = dfs(i + 1, 0);
            cout << (ans < 0 ? -1 : ans + i) << endl;
        }
    }
    return 0;
}