Single Number II

Given an array of integers, every element appears three times except for one. Find that single one.

Note:
Your algorithm should have a linear runtime complexity. Could you implement it without using extra memory?

int singleNumber(int A[], int n) {        int bitnum[32]={0};          int res=0;          for(int i=0; i<32; i++){              for(int j=0; j<n; j++){                  bitnum[i]+=(A[j]>>i)&1;              }              res|=(bitnum[i]%3)<<i;          }          return res;      }