PTA数据结构春季班第一单元

第一单元习题Maximum Subsequence Sum

Given a sequence of K integers { N​1, N​2 , …, N​K}. A continuous subsequence is defined to be { N​i​​ , N​i+1​​ , …, N​j } where 1≤i≤j≤K. The Maximum Subsequence is the continuous subsequence which has the largest sum of its elements. For example, given sequence { -2, 11, -4, 13, -5, -2 }, its maximum subsequence is { 11, -4, 13 } with the largest sum being 20.
Now you are supposed to find the largest sum, together with the first and the last numbers of the maximum subsequence.
Input Specification:
Each input file contains one test case. Each case occupies two lines. The first line contains a positive integer K (≤10000). The second line contains K numbers, separated by a space.

Output Specification:
For each test case, output in one line the largest sum, together with the first and the last numbers of the maximum subsequence. The numbers must be separated by one space, but there must be no extra space at the end of a line. In case that the maximum subsequence is not unique, output the one with the smallest indices i and j (as shown by the sample case). 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.
Sample Input:
10
-10 1 2 3 4 -5 -23 3 7 -21
Sample Output:
10 1 4

#include <stdio.h>
#include <time.h>
#include <iostream>

struct Result
{
	int Maxsum;
	int StartNum;
	int EndNum;
};

Result MaxSubseqSuml(int A[], int N)
{
	Result ret;

	bool flag = false;		//当前start位置前全是负数
	bool flagfor0 = false;  //当前start位置前全是0
	int NowStartNum = 0;
	int StartNum = 0;
	int EndNum = 0;
	int ThisSum = 0, Maxsum = -1;  //为了在Thissum>Maxsum中不带等于号
	


	for (int i = 0; i < N; i++) {
		ThisSum += A[i];       
		if (flagfor0 && ThisSum >= 0) { //前方是0(或被置为0)且当前和大于等于0
			NowStartNum = i;
			flagfor0 = false;
		}
		
		if (!flag && A[i] >= 0) {  //只要有一个正数flag就一直是true
			flag = true;
		}


		if (ThisSum >Maxsum)
		{
			StartNum = NowStartNum;
			EndNum = i;
			Maxsum = ThisSum;
		}
		if (ThisSum < 0) {
			ThisSum = 0; //abandon the negative subsequence
			flagfor0 = true;
		}

	}

	if (flag) {
		ret.Maxsum = Maxsum;
		ret.StartNum = StartNum;
		ret.EndNum = EndNum;
	}
	else
	{
		ret.Maxsum = 0;
		ret.StartNum = 0;
		ret.EndNum = N-1;
	}

	return ret;
}
int main()
{
	const int MaxSize = 10000;
	int A[MaxSize] = { 0 };
	int K;
	std::cin >> K;
	for (int i = 0; i < K; i++)
		std::cin >> A[i];

	Result res;
	res = MaxSubseqSuml(A, K);

	std::cout << res.Maxsum << " " << A[res.StartNum] << " " << A[res.EndNum];

	system("pause");
	return 0;
}