1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
#include <bits/stdc++.h>
#define mINF -1e9 + 11;
#define dINF 1e9 + 11
using namespace std;

constexpr int maxn = 50005;
int n, dp1, dp2, dp3, dp4, arr1[maxn], arr2[maxn], arr3[maxn], arr4[maxn];
bool w;

int main()
{
    cin.sync_with_stdio(0);
    cin >> n;
    for (int i = 0; i < n; ++i)
    {
        cin >> arr1[i];
        arr4[i] = arr3[i] = arr2[i] = arr1[i];
    }
    w = true; // 1 -> R ; 0 -> M
    // up
    for (int i = 1; i < n; ++i)
    {
        if (w)
        {
            w = !w;
            if (arr1[i - 1] < arr1[i])
                continue;
            else
            {
                arr1[i] = dINF;
                dp1++;
            }
        }
        else
        {
            w = !w;
            if (arr1[i - 1] > arr1[i])
                continue;
            else
            {
                arr1[i] = mINF;
                dp1++;
            }
        }
    }
    w = false;
    // down
    for (int i = 1; i < n; ++i)
    {
        if (w)
        {
            w = !w;
            if (arr2[i - 1] < arr2[i])
                continue;
            else
            {
                arr2[i] = dINF;
                dp2++;
            }
        }
        else
        {
            w = !w;
            if (arr2[i - 1] > arr2[i])
                continue;
            else
            {
                arr2[i] = mINF;
                dp2++;
            }
        }
    }
    w = true;
    for (int i = n - 2; i >= 0; --i)
    {
        if (w)
        {
            w = !w;
            if (arr3[i + 1] < arr3[i])
                continue;
            else
            {
                arr3[i] = dINF;
                dp3++;
            }
        }
        else
        {
            w = !w;
            if (arr3[i + 1] > arr3[i])
                continue;
            else
            {
                arr3[i] = mINF;
                dp3++;
            }
        }
    }
    w = false;
    for (int i = n - 2; i >= 0; --i)
    {
        if (w)
        {
            w = !w;
            if (arr4[i + 1] < arr4[i])
                continue;
            else
            {
                arr4[i] = dINF;
                dp4++;
            }
        }
        else
        {
            w = !w;
            if (arr4[i + 1] > arr4[i])
                continue;
            else
            {
                arr4[i] = mINF;
                dp4++;
            }
        }
    }
    cout << min(dp1, min(dp2, min(dp3, dp4)));
}