PAT甲级 1007 Maximum Subsequence Sum (25 分)

题意：

给定长度为n的序列，寻找最大字段和，按要求输出

思路：

滑动窗口，当前子段sum>=0时，右指针一直往右边滑动，如果当前序列sum变成的负数，那就把左指针往右滑动，直到序列sum变成非负数或者序列变空了；

 

ps：如果最大子段和为负，如下操作："If all the K numbers are negative, then its maximum sum is defined to be 0, and you are supposed to output the first and the last numbers of the whole sequence."

 

#include
#define FI first
#define SE second

using namespace std;
const int maxn = 1e4 + 7;
typedef long long ll;

int n, a[maxn];

int main() {
    scanf("%d", &n);
    for(int i = 1; i <= n; ++i) {
        scanf("%d", &a[i]);
    }
    int sum = a[1], max_ = a[1];
    int l = 1, r = 1;
    int ans1 = a[1], ans2 = a[1];
    while(r < n) {
        while(sum < 0 && l <= r) {
            sum -= a[l++];
        }
        r++;
        sum += a[r];
        if(sum > max_) {
            max_ = sum;
            ans1 = a[l];
            ans2 = a[r];
        }
    }
    if(max_ < 0) {
        max_ = 0; ans1 = a[1]; ans2 = a[n];
    }
    printf("%d %d %d", max_, ans1, ans2);
    return 0;
}