LeetCode: First Missing Positive

import static org.junit.Assert.assertSame;
/**
 * Description: Given an unsorted integer array, find the first missing positive integer.
 * <p/>
 * For example, Given [1,2,0] return 3, and [3,4,-1,1] return 2.
 * <p/>
 * Your algorithm should run in O(n) time and uses constant space.
 *
 * @author hzhou
 */
public class FirstMissingPositive {
    public int firstMissingPositive(int[] nums) {
        if (nums == null || nums.length < 1) {
            return 1;
        }
        for (int i = 0; i < nums.length; ) {
            int crt = nums[i];
            if (crt > 0 && crt <= nums.length && i + 1 != crt && nums[i] != nums[crt - 1]) {
                nums[i] = nums[crt - 1];
                nums[crt - 1] = crt;
            } else {
                i++;
            }
        }
        for (int i = 0; i < nums.length; i++) {
            if (i + 1 != nums[i]) {
                return i + 1;
            }
        }
        return nums.length + 1;
    }
    @Test
    public void test() {
        int[] nums = new int[]{1, 1};
        assertSame(firstMissingPositive(nums), 2);
    }
}