题意：
求最长回文串。。。但这个回文串要符合从中间到两头 逐个递减
解析：
在扩散的时候加一个判断就好了
#include <iostream> #include <cstdio> #include <sstream> #include <cstring> #include <map> #include <set> #include <vector> #include <stack> #include <queue> #include <algorithm> #include <cmath> #define rap(i, a, n) for(int i=a; i<=n; i++) #define rep(i, a, n) for(int i=a; i<n; i++) #define lap(i, a, n) for(int i=n; i>=a; i--) #define lep(i, a, n) for(int i=n; i>a; i--) #define rd(a) scanf("%d", &a) #define rlld(a) scanf("%lld", &a) #define rc(a) scanf("%c", &a) #define rs(a) scanf("%s", a) #define MOD 2018 #define LL long long #define ULL unsigned long long #define Pair pair<int, int> #define mem(a, b) memset(a, b, sizeof(a)) #define _ ios_base::sync_with_stdio(0),cin.tie(0) //freopen("1.txt", "r", stdin); using namespace std; const int maxn = 100010, INF = 0x7fffffff; int s[maxn], s_new[maxn<<1]; int p[maxn<<1]; int n; int init() { int len = n; s_new[0] = -1; s_new[1] = -2; int j = 2; for(int i=0; i<len; i++) s_new[j++] = s[i], s_new[j++] = -2; // s_new[j++] = -10; return j; } int manacher() { int len = init(); int max_len = -1; int id, mx = 0; for(int i=1; i<len; i++) { if(i < mx) p[i] = min(p[2 * id - i], mx - i); //因为这一步最初是不执行的 所以是依附于下边的扩散步骤 else p[i] = 1; while(s_new[i-p[i]] == s_new[i+p[i]]) { if(s_new[i+p[i]] != -2 && s_new[i+p[i] - 2] < s_new[i+p[i]]) //不符合就跳出 break; p[i]++; } if(mx < i + p[i]) { id = i; mx = i + p[i]; } max_len = max(max_len, p[i] - 1); } return max_len; } int T; int main() { rd(T); while(T--) { rd(n); rep(i, 0, n) rd(s[i]); printf("%d\n", manacher()); } return 0; }