FirstMissingPositive.java

public class FirstMissingPositive {
 /**
 *
 */
 public int firstMissingPositive(int[] nums) {
 if (nums == null || nums.length == 0) return 1;
 int i = 0;
 while (i < nums.length) {
 System.out.println("what ? :" + nums.length);
 if (nums[i] <= 0 || nums[i] > i + 1) i++;
 else if (nums[nums[i] - 1] != nums[i]) swap(nums, nums[i] - 1, i);
 else i++;
 }
 for (i = 0; i < nums.length; i++) {
 if (nums[i] != i + 1) return i + 1;
 }
 return nums.length + 1;
 }

 private void swap(int[] nums, int i, int j) {
 int tmp = nums[i];
 nums[i] = nums[j];
 nums[j] = tmp;
 }

 public static void main(String[] args) {
 FirstMissingPositive a = new FirstMissingPositive();
 System.out.println(a.firstMissingPositive(new int[] {3,4,-1,1}));
 }
}