public class FirstMissingPositive {

/**

* 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.

time : O(n)

space : O(1)

* @param nums

* @return

* [1,2,3]¶ÔӦϱê0£¬1£¬2

*/

public int firstMissingPositive(int[] nums) {

if (nums == null || nums.length == 0) return 1;

for (int i = 0; i < nums.length; i++) {

while (nums[i] > 0 && nums[i] <= nums.length && nums[nums[i] - 1] != nums[i]) {

int temp = nums[nums[i] - 1];

nums[nums[i] - 1] = nums[i];

nums[i] = temp;

}

}

for (int i = 0; i < nums.length; i++) {

if (nums[i] != i + 1) {

return i + 1;

}

}

return nums.length + 1;

}

}