41. First Missing Positive

41. First Missing Positive (Hard)

Given an unsorted integer array, find the smallest missing positive integer.

Example 1:

Input: [1,2,0]
Output: 3

Example 2:

Input: [3,4,-1,1]
Output: 2

Example 3:

Input: [7,8,9,11,12]
Output: 1

Note:

Your algorithm should run in O(n) time and uses constant extra space.

Related Topics:
Array

Similar Questions:

• Missing Number (Easy)
• Find the Duplicate Number (Medium)
• Find All Numbers Disappeared in an Array (Easy)
• Couples Holding Hands (Hard)

Solution 1.

// OJ: https://leetcode.com/problems/first-missing-positive/
// Author: github.com/lzl124631x
// Time: O(N)
// Space: O(1)
class Solution {
public:
    int firstMissingPositive(vector<int>& A) {
        int i, N = A.size();
        for (i = 0; i < N; ) {
            if (A[i] == i + 1 || A[i] < 1 || A[i] >= N + 1 || A[i] == A[A[i] - 1]) ++i;
            else swap(A[i], A[A[i] - 1]);
        }
        for (i = 0; i < N && A[i] == i + 1; ++i);
        return i + 1;
    }
};