http://poj.org/problem?

id=1681

求最少经过的步数使得输入的矩阵全变为y。

思路：高斯消元求出自由变元。然后枚举自由变元，求出最优值。

注意依据自由变元求其它解及求最优值的方法。

#include <stdio.h>
#include <algorithm>
#include <set>
#include <map>
#include <vector>
#include <math.h>
#include <string.h>
#define LL long long
#define _LL __int64using namespace std;
const int INF = 0x3f3f3f3f;char mapp[17][17];
int a[16*16][16*16];
int equ,var;
int x[16*16];
int free_x[16*16]; //保存自由变元，枚举求最优解
int free_num;void init()
{memset(a,0,sizeof(a));memset(x,0,sizeof(x));
}void debug()
{for(int i = 0; i < equ; i++){for(int j = 0; j < var+1; j++)printf("%d",a[i][j]);printf("\n");}
}int Gauss()
{int row,col,i,j;int max_r;row = col = 0;free_num = 0;while(row < equ && col < var){max_r = row;for(i = row+1; i < equ; i++){if( abs(a[i][col]) > abs(a[max_r][col]) )max_r = i;}if(max_r != row){for(j = col; j < var+1; j++)swap(a[max_r][j],a[row][j]);}if(a[row][col] == 0){free_x[ free_num++ ] = col; //该列相应的变量是自由元col++;continue;}for(i = row+1; i < equ; i++){if(a[i][col] == 0) continue;for(j = col; j < var+1; j++)a[i][j] ^= a[row][j];}row++;col++;}for(i = row; i < equ; i++)if(a[i][col] != 0)return -1; //无解if(row < var)return var-row; //返回自由变元的数目for(i = var-1; i >= 0; i--) //有唯一解{x[i] = a[i][var];for(j = i+1; j < var; j++)x[i] ^= (a[i][j] && x[j]);}return 0;
}void solve()
{int t = Gauss();if(t == -1){printf("inf\n");return;}else if(t == 0){int ans = 0;for(int i = 0; i < var; i++)ans += x[i];printf("%d\n",ans);return;}else{int ans = INF;int sta = (1<<t); //t个变量共同拥有sta个基础解int cnt;for(int i = 0; i < sta; i++){cnt = 0;//先给自由变元赋值for(int j = 0; j < t; j++){if((1<<j) & i){x[ free_x[j] ] = 1;cnt++;}elsex[ free_x[j] ] = 0;}//求出其它的解for(int j = var-t-1; j >= 0; j--){int l,k;for(k = j; k < var; k++)if(a[j][k])break; //先找到该行第一个不为0的数x[k] = a[j][var];for(l = k+1; l < var; l++)x[k] ^= (x[l] && a[j][l]);cnt += x[k];}ans = min(ans,cnt);}printf("%d\n",ans);return;}
}int main()
{int n,test;scanf("%d",&test);while(test--){init();scanf("%d",&n);equ = var = n*n;for(int i = 0; i < n; i++)scanf("%s",mapp[i]);for(int i = 0; i < n; i++){for(int j = 0; j < n; j++){if(mapp[i][j] == 'w')a[i*n+j][var] = 1;else a[i*n+j][var] = 0;}}for(int i = 0; i < equ; i++){int x = i/n;int y = i%n;for(int j = 0; j < var; j++){int xx = j/n;int yy = j%n;if( abs(x-xx) + abs(y-yy) <= 1)a[i][j] = 1;else a[i][j] = 0;}}solve();}return 0;
}