1 #include "stdafx.h"
2 #include <iostream>
3 #include <vector>
4 using namespace std;
5 int flag = 0;
6 long maxsum(const vector<int>& a)//时间复杂度O（n3）
7 {
8 if (flag == 0)
9 {
10 return 0;
11 }
12 long maxsum = 0;
13 for (int i = 0; i < a.size(); i++)
14 {
15 for (int j = i; j < a.size(); j++)
16 {
17 long thissum = 0;
18 for (int k = i; k <= j; k++)
19 {
20 thissum += a[k];
21
22 }
23 if (thissum>maxsum)
24 maxsum = thissum;
25 }
26 }
27 return maxsum;
28 }
29 long maxsum2(const vector<int>& a)//时间复杂度O（n2）
30 {
31 if (flag == 0)
32 {
33 return 0;
34 }
35 long maxsum = 0;
36 for (int i = 0; i < (int)a.size(); i++)
37 {
38 long thissum = 0;
39 for (int j = i; j < (int)a.size(); j++)
40 {
41 thissum += a[j];
42 if (thissum > maxsum)
43 maxsum = thissum;
44 }
45 }
46 return maxsum;
47 }
48 long maxsum3(const vector<int>& a)//时间复杂度O（n）
49 {
50 if (flag == 0)
51 {
52 return 0;
53 }
54 long maxsum = 0, thissum = 0;
55 for (int i = 0; i < (int)a.size(); i++)
56 {
57 thissum += a[i];
58
59 if (thissum > maxsum)
60 maxsum = thissum;
61 else if (thissum < 0)
62 {
63 thissum = 0;
64 }
65 }
66 return maxsum;
67 }
68 long maxsum4(const vector<int>& a,int left,int right) //时间复杂度n*log（n）
69 {
70 if (flag == 0)
71 {
72 return 0;
73 }
74 if (left == right)
75 {
76 if (a[left] > 0)
77 {
78 return a[left];
79 }
80 else
81 {
82 return 0;
83 }
84
85 }
86 int center = (left + right) / 2;
87 long maxleftsum = maxsum4(a, left, center);
88 long maxrightsum = maxsum4(a, center + 1, right);
89
90 long LBorder_maxsum = 0, LBorder_thissum = 0;
91 for (int i = center; i >= left; i--)
92 {
93 LBorder_thissum += a[i];
94 if (LBorder_thissum > LBorder_maxsum)
95 {
96 LBorder_maxsum = LBorder_thissum;
97 }
98 }
99 long RBorder_maxsum = 0, RBorder_thissum = 0;
100 for (int j = center + 1; j <= right; j++)
101 {
102 RBorder_thissum += a[j];
103 if (RBorder_thissum > RBorder_maxsum)
104 {
105 RBorder_maxsum = RBorder_thissum;
106 }
107 }
108
109 if (maxleftsum < maxrightsum)
110 {
111 maxleftsum = maxrightsum;
112 }
113 if (maxleftsum>(LBorder_maxsum + RBorder_maxsum))
114 {
115 return maxleftsum;
116 }
117 else
118 {
119 return (LBorder_maxsum + RBorder_maxsum);
120 }
121
122 }
123 int main(void)
124 {
125 int K;
126 int temp;
127
128 cin >> K;
129 vector<int>arry;
130 for (int i = 0; i < K; i++)
131 {
132 cin >> temp;
133 arry.push_back(temp);
134 if (temp >= 0)
135 {
136 flag = 1;
137 }
138
139 }
140 //cout <<maxsum(arry)<< endl;
141 //cout <<maxsum2(arry)<< endl;
142 //cout <<maxsum3(arry)<< endl;
143 cout << maxsum4(arry,0,arry.size()-1) << endl;
144 return 0;
145 }
