how much liquid will accumulate between the stacked boxes.

skyscrapers.c

#include <stdio.h>
#include <stdlib.h>
#define MIN(X,Y) ((X) < (Y) ? (X) : (Y))

void skyscrapers(int *h, int len) {
    int* pks = (int*)malloc(sizeof(int)*len);
    int i, p=h[0], v=0, sum=0;
    
    v = pks[0] = h[0];
    for (i = 1; i < len; ++i) {
        if (h[i] > v) v = h[i];
        pks[i] = v;
    }
    
    v = pks[len-1] = h[len-1];
    for (i = len-2; i >= 0; --i) {
        if (h[i] > v) v = h[i];
        pks[i] = MIN(pks[i], v);
    }
    
    for (i = 0; i < len; ++i) sum += pks[i] - h[i];
    printf("%d", sum);
    free(pks);
}