题目：

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.

思路：

不断进行交换，将值为 i 的数字交换到 (i - 1)th 的位置.

package array;public class FirstMissingPositive {    public int firstMissingPositive(int[] nums) {        int n;        if (nums == null || (n = nums.length) == 0) return 1;        for (int i = 0; i < n; ++i) {            int a = nums[i];            while (a > 0 && a <= n && nums[i] != i + 1 && nums[a - 1] != a) {                swap(nums, i, a - 1);                a = nums[i];            }        }                int j = 0;        while (j < n && nums[j] == j + 1){            ++j;        }        return j + 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) {        // TODO Auto-generated method stub        int[] nums = { 1,1 };        FirstMissingPositive f = new FirstMissingPositive();        System.out.println(f.firstMissingPositive(nums));    }}