PHP Program for Find the element that appears once

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Given an array where every element occurs three times, except one element which occurs only once. Find the element that occurs once. Expected time complexity is O(n) and O(1) extra space.
Examples :

Input: arr[] = {12, 1, 12, 3, 12, 1, 1, 2, 3, 3}
Output: 2
In the given array all element appear three times except 2 which appears once.

Input: arr[] = {10, 20, 10, 30, 10, 30, 30}
Output: 20
In the given array all element appear three times except 20 which appears once.

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

PHP

<?php 
  
function getSingle($arr, $n) 
{ 
    $ones = 0; $twos = 0 ; 
  
    $common_bit_mask; 
  
    // Let us take the example of  
    // {3, 3, 2, 3} to understand this 
    for($i = 0; $i < $n; $i++ ) 
    { 
        /* The expression "one & arr[i]"  
        gives the bits that are there in  
        both \'ones\' and new element from  
        arr[]. We add these bits to \'twos\'  
        using bitwise OR 
        Value of \'twos\' will be set as 0,  
        3, 3 and 1 after 1st, 2nd, 3rd  
        and 4th iterations respectively */
        $twos = $twos | ($ones & $arr[$i]); 
  
  
        /* XOR the new bits with previous  
        \'ones\' to get all bits appearing  
        odd number of times 
  
        Value of \'ones\' will be set as 3,  
        0, 2 and 3 after 1st, 2nd, 3rd and  
        4th iterations respectively */
        $ones = $ones ^ $arr[$i]; 
  
        /* The common bits are those bits  
        which appear third time. So these  
        bits should not be there in both  
        \'ones\' and \'twos\'. common_bit_mask  
        contains all these bits as 0, so  
        that the bits can be removed from  
        \'ones\' and \'twos\'  
  
        Value of \'common_bit_mask\' will be  
        set as 00, 00, 01 and 10 after 1st,  
        2nd, 3rd and 4th iterations respectively */
        $common_bit_mask = ~($ones & $twos); 
  
  
        /* Remove common bits (the bits  
        that appear third time) from \'ones\' 
              
        Value of \'ones\' will be set as 3,  
        0, 0 and 2 after 1st, 2nd, 3rd  
        and 4th iterations respectively */
        $ones &= $common_bit_mask; 
  
  
        /* Remove common bits (the bits  
        that appear third time) from \'twos\' 
  
        Value of \'twos\' will be set as 0, 3,  
        1 and 0 after 1st, 2nd, 3rd and 4th 
        itearations respectively */
        $twos &= $common_bit_mask; 
  
        // uncomment this code to see  
        // intermediate values  
        // printf (" %d %d n", ones, twos); 
    } 
  
    return $ones; 
} 
  
// Driver Code 
$arr = array(3, 3, 2, 3); 
$n = sizeof($arr); 
echo "The element with single " .  
                "occurrence is ",  
             getSingle($arr, $n); 
  
// This code is contributed by m_kit 
?> 

Please refer complete article on Find the element that appears once for more details!

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Last Updated :
22 Dec, 2021