daimayuan 707. 括号序列

题意：

给定括号字符串，问有多少个非空子串是合法括号序列

思路：

略。这题做不出来就remake吧

const signed N = 1e6 + 3;
int n, p[N]; char s[N];
int stk[N], top;
ll ans, f[N];

signed main() {
    iofast;
    cin >> s + 1; n = strlen(s + 1);

    for(int i = 1; i <= n; i++)
        if(s[i] == '(') stk[++top] = i;
        else if(top) p[i] = stk[top--]; //匹配的左括号位置

    for(int i = 1; i <= n; i++) if(p[i])
        f[i] = f[p[i]-1] + 1, ans += f[i];

    cout << ans;
}