Codeforces Round #663 (Div. 2) D.505

https://codeforces.ml/contest/1391/problem/D 

位运算，绝妙的一个题

#include <iostream>
#include <cstring>
#include <cstdio>
#include <vector>

using namespace std;

const int N = 500006;
char a[4][N];
vector<int> st;
int n,m;
vector<int> g[8];
int f[N][8];

int lowbit(int x){
    return x & -x;
}

void res1(){
    int ans1 = 0,ans2 = 0;
    for(int i = 0;i < st.size();i += 2){
        if(st[i] == 0 ||st[i] == 3) ans1++;
        if((st[i + 1] == 1 || st[i + 1] == 2) && i + 1 < st.size()) ans1++;
    }
    for(int i = 0;i < st.size();i += 2){
        if(st[i] == 1 || st[i] == 2) ans2++;
        if(i + 1 < st.size() && (st[i + 1] == 0 || st[i + 1] == 3)) ans2++;
    }
    printf("%d",min(ans1,ans2));
}

void res2(){
    g[0] = {5,2};g[1] = {4,3};
    g[2] = {7,0};g[3] = {1,6};
    g[4] = {1,6};g[5] = {0,7};
    g[6] = {4,3};g[7] = {5,2};
    for (int i = 0; i < 8; i++) {
        f[0][i] = lowbit(i ^ st[0]); 
    }
    for(int i = 1;i < st.size();i++)
        for(int j = 0;j < 8;j++){ 
            f[i][j] = min(f[i - 1][g[j][0]] + lowbit(j ^ st[i]), 
                            f[i - 1][g[j][1]] + lowbit(j ^ st[i]));
        }
    int ans = 0x3f3f3f3f;
    for(int i = 0;i < 8;i++) ans = min(ans,f[st.size() - 1][i]);
    printf("%d",ans);
}

int main(){
    scanf("%d%d",&n,&m);
    if(n >= 4 && m >= 4){
        puts("-1");
        return 0;
    }
    if(n == 1||m == 1){
        puts("0");
        return 0;
    }
    for(int i = 0;i < n;i++)
        scanf("%s",a[i]);
    for(int i = 0;i < m;i++){
        int x = 0;
        for(int j = 0;j < n;j++){
            if(j) x <<= 1;
            x += a[j][i] - '0';
        }
        st.push_back(x);
    }
    if(n == 2) res1();
    else res2();
    return 0;
}