2020ICPC小米网络赛第二场 A.2020

题目链接

题目描述
Bobo has a string $s_1\dots s_n$ of length $n$ consisting of only digits $0,1$, and $2$, and he wants to pick some disjoint subsequences which equal to $2020$, as many as possible.

Formally, Bobo would like to find $k$ quadrangle $(a_1,b_1,c_1,d_1),\dots ,(a_k,b_k,c_k,d_k)$ where

• $1\le a_i<b_i<c_i<d_i\le n$
• $s_{a_i}{s}_{b_i}{s}_{c_i}{s}_{d_i}=2020$
• $\{a_i,b_i,c_i,d_i\}\cap \{a_j,b_j,c_j,d_j\}=\varnothing$ for $i\ne j$

Find the maximum value of $k$.
输入描述:
The input consists of several test cases terminated by end-of-file.

The first line of each test case contains an integer n and the second line contains a string $s_1\dots s_n$.

• $1\le n\le 10^5$
• $s_i\in \{0,1,2\}$
• The sum of n does not exceed $10^6$ .

输出描述:
For each test case, print an integer which denotes the result.
示例1
输入

4
2222
7
2101210
8
22002200

输出

0
1
2

二分答案即可

#include

using namespace std;
const int N = 1e5 + 10;
int cnt[4], n;
char a[N];

inline bool check(int x) {
     
    memset(cnt, 0, sizeof(cnt));
    cnt[0] = x;
    for (int i = 1; i <= n; ++i) {
     
        if (a[i] == '2') {
     
            if (cnt[0])cnt[0]--, cnt[1]++;
            else if (cnt[2])cnt[2]--, cnt[3]++;
        } else if (a[i] == '0') {
     
            if (cnt[1])cnt[1]--, cnt[2]++;
            else if (cnt[3])cnt[3]--;
        }
    }
    if (cnt[0] || cnt[1] || cnt[2] || cnt[3])return false;
    return true;
}

int main() {
     
    while (~scanf("%d", &n)) {
     
        scanf("%s", a + 1);
        int l = 0, r = n >> 2;
        while (l < r) {
     
            int mid = (l + r + 1) >> 1;
            check(mid) ? (l = mid) : (r = mid - 1);
        }
        printf("%d\n", l);
    }
    return 0;
}