1126.积水

#include <cstdio>
#include <cstdlib>
#include <iostream>
using namespace std;

int a[5001];
int b[5001];
int h[5001];

int main(int argc, char* argv[]) {
    int n, ans = 0;
    int minh;

    scanf("%d", &n);
    for (int i = 1; i <= n; i++)
        scanf("%d", h + i);

    a[1] = h[1];
    for (int i = 2; i <= n; i++)
        a[i] = max(a[i - 1], h[i]);

    b[n] = h[n];
    for (int i = n - 1; i >= 1; i--)
        b[i] = max(b[i + 1], h[i]);

    for (int i = 2; i < n; i++) {
        minh = min(a[i - 1], b[i + 1]);
        if (minh > h[i])
            ans += minh - h[i];
    }
    printf("%d\n", ans);

    return 0;
}