isSumOf2.go

package main

import (
	"fmt"
	"sort"
)

// Implementation of sum of two ints. Do any two ints
// in the given slice sum up to the target? Leverages
// sorting O(n logn) and two pointers. If the sum of the
// pointed to ints are less than the target, then we move
// the left pointer right. If the sum is greater than the target,
// then we move the right pointer left until either we find
// a pair that sums to the target or the two pointers meet.
// A naaive approach would be a nested loop in order to test
// every combination. That, of course, would be O(n^2) time, much
// worse than sorting a slice.
func isSumOf2(xs []int, target int) bool {
	sort.Ints(xs)
	leftPointer := 0
	rightPointer := len(xs) - 1
	for leftPointer < rightPointer {
		sum := xs[leftPointer] + xs[rightPointer]
		if sum > target {
			rightPointer = rightPointer - 1
		} else if sum < target {
			leftPointer = leftPointer + 1
		} else {
			return true
		}
	}
	return false
}
func main() {
	xs := []int{5, 2, 3, 1}
	fmt.Println(isSumOf2(xs, 4))

	xs = []int{5, 2, 3, 1}
	fmt.Println(isSumOf2(xs, 9))

	xs = []int{1}
	fmt.Println(isSumOf2(xs, 9))

	xs = []int{}
	fmt.Println(isSumOf2(xs, 9))
}