Educational Codeforces Round 53 (Rated for Div. 2) C. Vasya and Robot

  先提前处理求出u，d，l，r的前缀数组，然后 二分修改区间的长度即可

#include<bits/stdc++.h>
using namespace std;
const int maxn=2e5+10;
int n,x7,y7,x2,y2,preu[maxn],pred[maxn],prel[maxn],prer[maxn];
bool check(int len)
{
    int tmp,st;
    for(int ed=len;ed<=n;ed++)
    {
        st=ed-len+1;
        x2=prer[st-1]-prel[st-1]+prer[n]-prer[ed]-(prel[n]-prel[ed]);
        y2=preu[st-1]-pred[st-1]+preu[n]-preu[ed]-(pred[n]-pred[ed]);
        tmp=abs(x7-x2)+abs(y7-y2);
        if(len>=tmp&&(len-tmp)%2==0)
            return true;
    }
    return false;
}
int main()
{
    int minn=-1,low,high,mid;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
    {
        char tmp;
        scanf(" %c",&tmp);
        prer[i]=prer[i-1];
        prel[i]=prel[i-1];
        preu[i]=preu[i-1];
        pred[i]=pred[i-1];
        if(tmp=='R')
            prer[i]+=1;
        else
            if(tmp=='L')
                prel[i]+=1;
            else
                if(tmp=='U')
                    preu[i]+=1;
                else
                    pred[i]+=1;

    }
    scanf("%d %d",&x7,&y7);
    if((prer[n]-prel[n]==x7)&&(preu[n]-pred[n]==y7))
    {
        printf("0
");
        return 0;
    }
    low=1,high=n;
    while(low<=high)
    {
        mid=(low+high)/2;
        if(check(mid))
        {
            minn=mid;
            high=mid-1;
        }
        else
            low=mid+1;
    }
    printf("%d
",minn);
    return 0;
}