【HDU】5248-序列变换（贪心+二分）「建议收藏」

二分枚举长度改变的长度即可了

#include<cstdio>
#include<cstring>
#include<algorithm>
using namespace std;
const int  INF = 3000000;
const int maxn = 100005;
int n,arr[maxn],arr2[maxn];
bool solve(int x){
    memcpy(arr2,arr,sizeof(arr));
    for(int i = 1; i <= n; i++){
        if(arr2[i] > arr2[i - 1]){
            arr2[i] = max(arr2[i - 1] + 1,arr2[i] - x);
        }
        else{
            if(arr2[i] + x <= arr2[i - 1]) return false;
            arr2[i] = arr2[i - 1] + 1;
        }
    }
    return true;
}
int main(){
    int T,Case = 1;
    scanf("%d",&T);
    arr[0] = -INF;
    while(T--){
        scanf("%d",&n);
        for(int i = 1; i <= n; i++)
            scanf("%d",&arr[i]);
        int l = 0,r = INF;
        int ans = n;
        while(l <= r){
            int mid = (l + r) >> 1;
            if(solve(mid)){
                ans = mid;
                r = mid - 1;
            }
            else
                l = mid + 1;
        }
        printf("Case #%d:\n",Case++);
        printf("%d\n",ans);
    }
    return 0;
}