C Exercises: First missing +ve integer in unsorted array

Last update on August 09 2025 12:47:39 (UTC/GMT +8 hours)

28. Descending Prime Numbers (Strictly Decreasing Digits) Variants

Write a C program to find the first missing positive integer in a given unsorted integer array.

C Code:

//https://github.com/begeekmyfriend/leetcode/blob/master/041_first_missing_positive/missing_positive.c
#include <stdio.h>
#include <stdlib.h>
static inline void swap_val(int *x, int *y)
{
    int tmp = *x;
    *x = *y;
    *y = tmp;
}

static int first_Missing_Positive_num(int* nums, int nums_size)
{
    if (nums_size < 1) {
        return 1;
    }

    int i = 0;
    while (i < nums_size) {
        if (nums[i] != i + 1 && nums[i] > 0 && nums[i] <= nums_size && nums[i] != nums[nums[i] - 1]) {
            swap_val(nums + i, nums + nums[i] - 1);
        } else {
            i++;
        }
    }

    for (i = 0; i < nums_size; i++) {
        if (nums[i] != i + 1) break;
    }
    return i + 1;
}

int main()
{
    int i, count = 4;
    int nums[4] = {3,4,-1,1};
      
    printf("%d\n", first_Missing_Positive_num(nums, count));
    return 0;
}

Sample Output:

Flowchart:

For more Practice: Solve these Related Problems:

Write a C program to generate all prime numbers with digits in strictly descending order.
Write a C program to count and list primes that have strictly decreasing digits from a given range.
Write a C program to display descending prime numbers and compare their distribution with ascending ones.
Write a C program to compute the average of prime numbers with strictly descending digits.

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