Given an array of integers, and a number ‘sum’, find the number of pairs of integers in the array whose sum is equal to ‘sum’.
Examples:
Input : arr[] = {1, 5, 7, -1},
sum = 6
Output : 2
Pairs with sum 6 are (1, 5) and (7, -1)
Input : arr[] = {1, 5, 7, -1, 5},
sum = 6
Output : 3
Pairs with sum 6 are (1, 5), (7, -1) &
(1, 5)
Input : arr[] = {1, 1, 1, 1},
sum = 2
Output : 6
There are 3! pairs with sum 2.
Input : arr[] = {10, 12, 10, 15, -1, 7, 6,
5, 4, 2, 1, 1, 1},
sum = 11
Output : 9
Expected time complexity O(n)
Naive Solution – A simple solution is to traverse each element and check if there’s another number in the array which can be added to it to give sum.
PHP
<?php // PHP implementation of simple // method to find count of // pairs with given sum. // Returns number of pairs in // arr[0..n-1] with sum equal // to 'sum' function getPairsCount($arr, $n, $sum) { // Initialize result $count = 0; // Consider all possible pairs // and check their sums for ($i = 0; $i < $n; $i++) for ($j = $i + 1; $j < $n; $j++) if ($arr[$i] + $arr[$j] == $sum) $count++; return $count; } // Driver Code $arr = array(1, 5, 7, -1, 5) ; $n = sizeof($arr); $sum = 6; echo "Count of pairs is " , getPairsCount($arr, $n, $sum); // This code is contributed by nitin mittal. ?> |
Output
Count of pairs is 3
Time Complexity: O(n2)
Auxiliary Space: O(1)
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