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.

public class Solution {
public int firstMissingPositive(int[] A) {
if(A.length==0) return 1;
for(int i=0;i<A.length;i++){
while(A[i]!=i+1){
if(A[i]<=0 || A[i]>A.length || A[i]==A[A[i]-1]) break;
swap(A,i,A[i]-1);
}
}
for(int i=0;i<A.length;i++){
if(A[i]!=i+1) return i+1;
}
return A.length+1;
}
public void swap(int []A,int i,int j){
if(i!=j){
A[i]=A[i]^A[j];
A[j]=A[i]^A[j];
A[i]=A[j]^A[i];
}
}
}

思路：桶排序，把k个数放到k+1个桶里，必然有一个为空。每一次交换都把一个数放到正确的位置。然后遍历找到第一个不正确的。