HDU 3336 Count the string （kmp+dp）

题意:

求串的所有前缀在串中出现次数的总和

分析:

dp[i]:=前i个字符中出现以第i个字符结尾的前缀的次数
dp[i]=dp[nxt[i]]+1
即第i个字符结尾的前缀数等于以第next[i]个字符为结尾的前缀数加上它自己本身,这俩是同一个字符
　 i: 1 2 3 4
　 c: a b a b
dpi: 1 1 2 2
dp数组正好是原来那个倒过来的

代码:

////  Created by TaoSama on 2015-10-30//  Copyright (c) 2015 TaoSama. All rights reserved.////#pragma comment(linker, "/STACK:1024000000,1024000000")#include <algorithm>#include <cctype>#include <cmath>#include <cstdio>#include <cstdlib>#include <cstring>#include <iomanip>#include <iostream>#include <map>#include <queue>#include <string>#include <set>#include <vector>using namespace std;#define pr(x) cout << #x << " = " << x << "  "#define prln(x) cout << #x << " = " << x << endlconst int N = 2e5 + 10, INF = 0x3f3f3f3f, MOD = 1e4 + 7;int n, nxt[N], dp[N];char s[N];int getNxt() {    nxt[0] = -1;    int i = 0, j = -1;    while(i < n) {        if(j == -1 || s[i] == s[j]) nxt[++i] = ++j;        else j = nxt[j];    }}void add(int& x, int y) {if((x += y) >= MOD) x %= MOD;}int main() {#ifdef LOCAL    freopen("C:\\Users\\TaoSama\\Desktop\\in.txt", "r", stdin);//  freopen("C:\\Users\\TaoSama\\Desktop\\out.txt","w",stdout);#endif    ios_base::sync_with_stdio(0);    int t; scanf("%d", &t);    while(t--) {        scanf("%d%s", &n, s);        getNxt();        int ans = 0;        for(int i = 1; i <= n; ++i) {            dp[i] = dp[nxt[i]] + 1;            add(ans, dp[i]);        }        printf("%d\n", ans);    }    return 0;}