Leetcode 41: First Missing Positive

leetcode_41.cpp

#include <iostream>
#include <vector>
#include <climits>

using namespace std;

int firstMissingPositive(vector<int>& nums) {
    //first pass. replace all vals>n and <=0 with INT_MAX
    int n = nums.size();
    for (int i=0; i<n; i++) {
        if (nums[i] > n || nums[i] <= 0)
            nums[i] = INT_MAX;
    }

    //add extra value, as we need it for elem indexes 1 -> n.
    nums.push_back(INT_MAX);

    //next pass to flag values we have by setting negatives in indexes of those values
    for (int i=0; i<n; i++) {
        if (abs(nums[i]) == INT_MAX)
            continue;

        if (nums[abs(nums[i])] > 0)
            nums[abs(nums[i])] *= -1;
    }

    //final pass to find where we have not flagged a value (by -ve) and that index indicates the first missing positive.
    int i=1;
    for ( ; i<=n; i++) {
        if (nums[i] > 0)
            break;
    }

    return i;
}

int main() {
    vector<int> nums {1,2,4,8,5,-3};
    int res = firstMissingPositive(nums);
    cout << res << endl;
    return 0;
}