First Missing Positive

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.

高效算法：O(n) 时间复杂度，O(1)空间复杂度

实现：

1. int firstMissingPositive(int A[], int n) {
2. if(n<=0) return 1;
4. for(int j=0;j<n;) {
5. if(A[j]>0 && A[j]<n && A[j]!=j && A[A[j]]!=A[j]) {
6. swap(A[j],A[A[j]]);
7. }else {
8. j++;
9. }
10. }
11. for(int j=1;j<n;j++) {
12. if(A[j]!=j) return j;
13. }
14. if(A[0] == n) return n+1;
15. else return n;
16. }