分治法求最大子段和
问题分析：（直接截的PPT）
代码如下：
1 #include <iostream> 2 #include <math.h> 3 using namespace std; 4 //寻找最大子段和 5 int MaxSum(int a[], int left, int right) { 6 int sum = 0, midsum = 0, leftsum = 0, rightsum = 0; 7 if (left == right) sum = a[left]; 8 else { 9 //划分 10 int center, lefts, rights, s1, s2, i; 11 center = (left + right) / 2; 12 leftsum = MaxSum(a, left, center); 13 rightsum = MaxSum(a, center+1, right); 14 //情况3 15 s1 = 0; 16 lefts = 0; 17 for (i = center; i >= left; i--) { 18 lefts = lefts + a[i]; 19 if (lefts > s1) s1 = lefts; 20 } 21 s2 = 0; 22 rights = 0; 23 for (i = center + 1; i <= right; i++) { 24 rights = rights + a[i]; 25 if (rights > s2) s2 = rights; 26 } 27 midsum = s1 + s2; 28 //寻找最大值 29 if (leftsum > rightsum) sum = leftsum; 30 else sum = rightsum; 31 if (sum < midsum) sum = midsum; 32 } 33 return sum; 34 } 35 int main() { 36 int a[10]; 37 cout << "请依次输入十个数" << endl; 38 int i, sum; 39 for (i = 0; i < 10; i++) cin >> a[i]; 40 cout << "输入完成！" << endl; 41 int left = 0, right = 9; 42 sum = MaxSum(a, left, right); 43 cout << "最大子段和为" << sum << endl; 44 return 0; 45 }
时间复杂度：O(nlog2n)
