UVA 12467

UVA 12467 - Secret Word

kmp+二分，二分枚举答案的长度，用kmp判断是否合法

#include <bits/stdc++.h>

using namespace std;
#define rint register int
#define rll register long long
#define long long ll
const int maxn = 1e6 + 100;
char a[maxn], b[maxn];
int pi[maxn], f[maxn];

bool check(int mid, int n) {
    pi[mid] = 0;
    for (rint i = mid + 1, j = 0; i <= n; i++) {
        while (j > 0 && a[i] != a[mid + j]) j = pi[j + mid - 1];
        if (a[i] == a[mid + j]) j++;
        pi[i] = j;
    }
    for (int i = 1, j = 0; i <= n; i++) {
        while (j > 0 && (j == n || b[i] != a[j + mid])) j = pi[j + mid - 1];
        if (b[i] == a[j + mid]) j++;
        f[i] = j;
        if (f[i] == n - mid + 1) return true;
    }
    return false;
}

int main() {
//    freopen("in.txt", "r", stdin);
    rint _;
    scanf("%d", &_);
    while (_--) {
        scanf("%s", b + 1);
        rint n = strlen(b + 1);
        rint tot = 0;
        for (int i = n; i >= 1; i--) {
            a[++tot] = b[i];
        }
        a[n + 1] = '';
        rint l = 1, r = n, mid;
        while (l < r) {
            mid = (l + r) >> 1;
            if (check(mid, n)) r = mid;
            else l = mid + 1;
        }
        for (rint i = l; i <= n; i++) {
            printf("%c", a[i]);
        }
        printf("
");
    }
    return 0;
}