[LeetCode 141] First Missing Positive

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

For example,

Given

[1,2,0]

return

3

,

and

[3,4,-1,1]

return

2

.

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

solution:

store nums[i] at index nums[i]-1

public int firstMissingPositive(int[] nums) {
        for(int i=0;i<nums.length;i++){
            while(nums[i]<= nums.length && nums[i]>0 && nums[nums[i] -1]!= nums[i]) {
                int temp = nums[i];
                nums[i] = nums[nums[i] - 1];
                nums[temp - 1] = temp;
            }
        }
        for(int i=0;i<nums.length;i++){
            if(nums[i]!= i+1) return i+1;
        }
        return nums.length +1;
    }