[leetcode 41] 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.

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