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Leetcode041-firstMissingPositive

Posted on Edited on In Views: Valine:
Solution Report of LeetCode Acceptted

Description

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

Example

Example 1:

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

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

Example 3:
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Input: [7,8,9,11,12]
Output: 1

Note:

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

Solution

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class Solution {
    public int firstMissingPositive(int[] nums) {
        if (nums == null || nums.length == 0) return 1;
        
        int i = 0;
        // If the current value is in the range of (0,length) and it's not at its correct position, 
        // swap it to its correct position.
        // Else just continue;
        while (i < nums.length){
            if (nums[i] == i + 1 || nums[i] <= 0 || nums[i] > nums.length || nums[i] == nums[nums[i] - 1]) i++;
            else swap(nums, i, nums[i] - 1);
        }
        for (i = 0; i < nums.length; i++)
            if (nums[i] != i + 1) break;
        return i+1;
    }
    
    private void swap(int[] nums, int x, int y){
        int tmp = nums[x];
        nums[x] = nums[y];
        nums[y] = tmp;
    }
}