41. First Missing Positive

Given an unsorted integer array, find the first missing positive integer.
For example,Given[1,2,0]
return3
,and[3,4,-1,1]
return2.
Your algorithm should run in O(n) time and uses constant space.

public class Solution {
    public int firstMissingPositive(int[] nums) {
        int n = nums.length;
        for(int i=0;i<n;i++)
        {
            if(nums[i]>0&&nums[i]<=n)
            {
                if(nums[i]-1!=i&&nums[nums[i]-1]!=nums[i])
                  {
                      int temp = nums[nums[i]-1];    //此处要注意交换顺序！！！千万不要在交换的过程中改变变量的值
                      nums[nums[i]-1] = nums[i];
                      nums[i] = temp;
                      i--;
                  }
            }
        }
        for(int i=0;i<n;i++)
        {
            if(nums[i]!=i+1)
              return i+1;
        }
        return n+1;
        
    }
}