Sample Code

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#include <stdio.h>
#define MAXN 128
int main() {
    int n, p[MAXN], is_unknown[MAXN], previous = 100, sum = 0;
    scanf("%d", &n);
    // Set every -1 to as large as it can possibly be,
    // while ensuring the percentages are non-increasing.
    for (int i = 0; i < n; i++) {
        scanf("%d", &p[i]);
        if (p[i] == -1) {
            p[i] = previous;
            is_unknown[i] = 1;
        } else if (p[i] > previous) {
            // This percentage is more than that to its left.
            for (int i = 0; i < n; i++) printf("%d ", -1);
            printf("\n");
            return 0;
        }
        previous = p[i];
        sum += p[i];
    }
    // Go from right to left, decreasing percentages where possible,
    // ensuring each percentage is no less than that to its right.
    // Only do this while sum > 100. 
    previous = 1;
    for (int i = n-1; i >= 0; i--) {
        while (is_unknown[i] && sum > 100 && p[i] > previous) {
            p[i]--;
            sum--;
        }
        previous = p[i];
    }
    // If sum is less than 100, we can't increase it any more.
    // If sum is more than 100, we can't decrease it any more.
    if (sum == 100) {
        for (int i = 0; i < n; i++) printf("%d ", p[i]);
        printf("\n");
    } else {
        for (int i = 0; i < n; i++) printf("%d ", -1);
        printf("\n");
    }
}