D - Longest X

https://atcoder.jp/contests/abc229/tasks/abc229_d

参考： https://zhuanlan.zhihu.com/p/441875505

思路

使用acc累计数组，统计每个位置之前的.的数目

设置滑动窗口最大化字串长度，使得窗口内k次替换都被执行， 从头到尾进行统计，记录最大字串长度。

Code

string s;
int k;
int acc[200005];
 
int main()
{
    cin >> s;
    cin >> k;
 
    memset(acc, 0, sizeof(acc));
 
    int n = s.size();
    for(int i=0; i<n; i++){
        char one = s[i];
 
        int rep = 0;
        if (one == '.'){
            rep = 1;
        }
 
        acc[i+1] = acc[i] + rep;
    }
 
    int maxlen = 0;
    int i=1, j=1;
    while(true){
        if (i > n || j > n){
            break;
        }
 
//      for k == 0, it is possible to be this：
//      i > j
//        if (i > j){
//            break;
//        }
 
        while(acc[j]-acc[i-1]<=k){
            j++;
            if (j>n){
                break;
            }
        }
 
        // for k == 0, and ...., j may be 1, so it is need to updated.
        if (i > j){
            j = i;
            continue;
        }
 
        // back to last element X,
        // from i to it (inclusively) k operation done.
        j--;
 
        maxlen = max(maxlen, j-i+1);
 
        j++;
 
        i++;
    }
 
    cout << maxlen << endl;
 
    return 0;
}