UVA10913 - Walking on a Grid
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题意:一个N * N的网格,每个格子里面有数值,求从(1, 1)到(N, N)相加数值的最大值。不过是有条件规定的,1、只能走三个方向向左,向右,向下,并且不能超出网格范围。2、每个网格只能走一次。3、所经过的负数的个数是有限定的
思路:本来只用二维+dfs,但是发现实现起来有挺多地方不对。看了网上的思路,四维dp+dfs,d[i][j][k][l]表示在(i, j)网格经过K个负数(i是用来记录每走一次的方向)的数值。
#include <iostream>#include <cstdio>#include <cstring>#include <algorithm>using namespace std;typedef long long ll;const int INF = 0x3f3f3f3f;const int MAXN = 85;const int dx[] = {0, 0, 1};const int dy[] = {-1, 1, 0};int n, k;int g[MAXN][MAXN], vis[MAXN][MAXN]; ll d[MAXN][MAXN][10][10], ans;void init() { for (int i = 1; i <= n; i++) for (int j = 1; j <= n; j++) scanf("%d", &g[i][j]); for (int i = 0; i <= n; i++) for (int j = 0; j <= n; j++) for (int kk = 0; kk < 6; kk++) for (int l = 0; l < 3; l++) d[i][j][kk][l] = -INF; memset(vis, 0, sizeof(vis)); vis[1][1] = 1; ans = -INF;}void dfs(int x, int y, int cnt, ll sum) { if (cnt > k) return; if (x == n && y == n) { ans = max(ans, sum); return; } for (int i = 0; i < 3; i++) { int tx = x + dx[i]; int ty = y + dy[i]; if (tx > 0 && tx <= n && ty > 0 && ty <= n && !vis[tx][ty]) { vis[tx][ty] = 1; if (g[tx][ty] >= 0 && d[tx][ty][cnt][i] < sum + g[tx][ty]) { d[tx][ty][cnt][i] = sum + g[tx][ty]; dfs(tx, ty, cnt, d[tx][ty][cnt][i]); } else if (g[tx][ty] < 0 && d[tx][ty][cnt + 1][i] < sum + g[tx][ty]){ d[tx][ty][cnt + 1][i] = sum + g[tx][ty]; dfs(tx, ty, cnt + 1, d[tx][ty][cnt + 1][i]); } vis[tx][ty] = 0; } } }int main() { int t = 1; while (scanf("%d %d", &n, &k) && n || k) { init(); if (g[1][1] >= 0) { dfs(1, 1, 0, g[1][1]); } else dfs(1, 1, 1, g[1][1]); printf("Case %d: ", t++); if(ans == -INF) printf("impossible\n"); else printf("%lld\n", ans); } return 0;}
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