First Missing Positive Integer

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Question

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

Example

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

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

Solution

Actually, this approach is quite tricky to me. We have to maintain the sequence like index0 -> 1, index1 -> 2, etc, when we see the element didn’t meet the requirement we have to swap until we cannot move or meet the requirement. nums[i] should be nums[i] - 1(index) position.

Code

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// time:O(n)
// space:O(1)
public int firstMissingPositive(int[] nums) {
        if (nums == null || nums.length == 0) return 1;
        for (int i = 0; i < nums.length; i++) {
            while (nums[i] - 1 >= 0 && nums[i] - 1 < nums.length 
                   && nums[i] != nums[nums[i] - 1]) {
                exch(nums, nums[i] - 1, i);
            }
        }
        for (int i = 0; i < nums.length; i++) {
            if (nums[i] != i + 1) return i + 1;
        }
        return nums.length + 1;
    }
    
    public void exch(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }