1 // 652939    zhuli19901106    1367    Accepted    点击此处查看所有case的执行结果    1020KB    1192B    10MS
 2 // 201311172259
 3 #include <cstdio>
 4 using namespace std;
 5
 6 bool check_postorder(const int a[], int left, int right)
 7 {
 8     if(a == NULL || left < 0 || right < 0 || left > right){
 9         return false;
10     }
11
12     if(left == right){
13         return true;
14     }
15
16     int i;
17
18     i = left;
19     while(i <= right - 1 && a[i] < a[right]){
20         ++i;
21     }
22     if(i == right){
23         // right substree is empty
24         return check_postorder(a, left, right - 1);
25     }else if(i == left){
26         // left substree is empty
27         for(; i <= right - 1; ++i){
28             if(a[i] < a[right]){
29                 return false;
30             }
31         }
32         return check_postorder(a, left, right - 1);
33     }else{
34         int pos = i;
35         for(; i <= right - 1; ++i){
36             if(a[i] < a[right]){
37                 return false;
38             }
39         }
40         return check_postorder(a, left, pos - 1) && check_postorder(a, pos, right - 1);
41     }
42 }
43
44 int main()
45 {
46     const int MAXN = 10005;
47     int a[MAXN];
48     int n, i;
49
50     while(scanf("%d", &n) == 1){
51         for(i = 0; i < n; ++i){
52             scanf("%d", &a[i]);
53         }
54         if(check_postorder(a, 0, n - 1)){
55             printf("Yes\n");
56         }else{
57             printf("No\n");
58         }
59     }
60
61     return 0;
62 }