41. First Missing Positive

cl_303303303

　　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.
　　example [-1,-2, 1,3];
　　i ++;  
　　i++ ;
　　i = 2  nums[2] = 1;  nums[nums[2] -1] = nums[0] exchange with nums[2] -> [1, -2, -1, 3]
　　i++;
　　i=3 nums[3] = 3 -> nums[2] exchange with nums[3] - > [1,-2,3,-1];
　　out of loop
　　anther loop  nums[0]= 1 got, nums[1] =-2 return i+1 -> 2



public class Solution {
public int firstMissingPositive(int[] nums) {
int i = 0 ;
while(i < nums.length){
if(nums == i + 1 || nums <= 0 || nums > nums.length) i++;
else if(nums != nums[nums-1]) swap(nums, i, nums-1);
else i++;
}
i = 0;
while(i < nums.length && i+1 == nums) i++;
return i+1;
}
public void swap(int[] nums, int i, int j){
int temp = nums;
nums = nums[j];
nums[j] = temp;
}
}