CodeForces 451D Count Good Substrings

哎，最近都在做图论，没有练DP，现在一遇到DP就不会了= =

因为有合并这个操作，所以只要是首位相同的字符串肯定是能够构成good串的，那么只要统计在奇数位上出现的0，1的个数和偶数位数，随便递推一下就出来了

#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <climits>
#include <string>
#include <iostream>
#include <map>
#include <cstdlib>
#include <list>
#include <set>
#include <queue>
#include <stack>

using namespace std;

typedef long long LL;
const int maxn = 100005;
LL cntodd[2],cnteven[2];
char buf[maxn];

int main() {
    scanf("%s",buf + 1);
    int len = strlen(buf + 1);
    LL sumodd = 0,sumeven = 0;
    for(int i = 1;i <= len;i++) {
        int now = buf[i] - 'a';
        if(i & 1) cntodd[now]++;
        else cnteven[now]++;
        if(i & 1) {
            sumodd += cntodd[now];
            sumeven += cnteven[now];
        } else {
            sumodd += cnteven[now];
            sumeven += cntodd[now];
        }
    }
    cout << sumeven << " " << sumodd << endl;
    return 0;
}