Question

for starters, I did have a look at these questions:

Given an array of integers where some numbers repeat 1 time, some numbers repeat 2 times and only one number repeats 3 times, how do you find the number that repeat 3 times

Algorithm to find two repeated numbers in an array, without sorting

this one different:

given an unsorted array of integers with one unique number and the rest numbers repeat 3 times,
i.e.:

  {4,5,3,  5,3,4, 1, 4,3,5 }

we need to find this unique number in O(n) time and O(1) space

NOTE: this is not a homework, just I an nice question I came across

Answer 1

What about this one:

Idea: do bitwise addition mod 3

#include <stdio.h>

int main() {
    int a[] = { 1, 9, 9, 556, 556, 9, 556, 87878, 87878, 87878 };
    int n = sizeof(a) / sizeof(int);
    int low = 0, up = 0;

    for(int i = 0; i < n; i++) {
        int x = ~(up & a[i]);
        up &= x;
        x &= a[i];
        up |= (x & low);
        low ^= x;
    }
    printf("single no: %d\n", low);
}