Poj 2796 单调栈

关于单调栈的性质，和单调队列基本相同，只不过单调栈只使用数组的尾部，　类似于栈。

Accepted Code:

 1 /*************************************************************************  2  > File Name: 2796.cpp  3  > Author: Stomach_ache  4  > Mail: [email protected]  5  > Created Time: 2014年07月21日 星期一 22时45分56秒  6  > Propose:  7  ************************************************************************/

 8 #include <cmath>

 9 #include <string>

10 #include <cstdio>

11 #include <fstream>

12 #include <cstring>

13 #include <iostream>

14 #include <algorithm>

15 using namespace std; 16 

17 typedef long long LL; 18 const int maxn = 100005; 19 int n; 20 // b[i] holds the smallest index where a[b[i]]...a[i] are not larger than a[i], and e[i] is similar, just extends to the right of a[i]

21 int a[maxn], b[maxn], e[maxn], st[maxn]; 22 LL sum[maxn]; //prefix sum 

23 

24 int

25 main(void) { 26 #ifndef ONLINE_JUDGE 27       freopen("in.txt", "r", stdin); 28 #endif

29       while (~scanf("%d", &n)) { 30           for (int i = 1; i <= n; i++) scanf("%d", a+i); 31         for (int i = 1; i <= n; i++) b[i] = e[i] = i; 32         sum[0] = 0; 33         for (int i = 1; i <= n; i++) { 34             sum[i] = sum[i-1] + a[i]; 35  } 36         // the top of no decreasing stack

37         int top = 0; 38         for (int i = 1; i <= n; i++) { 39               if (!top || a[i] > a[st[top-1]]) { 40                 // push an element to stack

41                   st[top++] = i; 42             } else { 43                   while (top > 0 && a[st[top-1]] > a[i]) { 44                       // update e[st[top-1]]

45                       e[st[top-1]] = i - 1; 46                     // update b[i]

47                     b[i] = b[st[top-1]]; 48                     // pop the toppest element of stack 

49                       top--; 50  } 51                 if (!top || a[st[top-1]] != a[i]) { 52                       // push

53                       st[top++] = i; 54                 } else if(a[st[top-1] == a[i]]) { 55                       // update b[i]

56                       b[i] = b[st[top-1]]; 57  } 58  } 59  } 60         // update e[i] of those elements that still in the stack 

61         for (int i = 0; i < top; i++) e[st[i]] = n; 62         // find the answer

63         LL ans = -1, bb, ee; 64         for (int i = 1; i <= n; i++) { 65               if (ans < a[i] * (sum[e[i]]-sum[b[i]-1])) { 66                   ans = a[i] * (sum[e[i]]-sum[b[i]-1]); 67                 bb = b[i]; 68                 ee = e[i]; 69  } 70  } 71         printf("%lld\n%lld %lld\n", ans, bb, ee); 72  } 73 

74     return 0; 75 }