Array Algorithm:Two Number Sum

算法刷题系列 Algorithm

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Difficulty：easy

Two Number Sum

Write a function that takes in a non-empty array of distinct integers and an integer representing a target sum. If any two numbers in the input array sum up to the target sum, the function should return them in an array, in any order. If no two numbers sum up to the target sum, the function should return an empty array.
Note that the target sum has to be obtained by summing two di!erent integers in the array; you can’t add a single integer to itself in order to obtain the target sum.
You can assume that there will be at most one pair of numbers summing up to the target sum.

Solution 1:
time complexity : O(n^2)

def twoNumberSum(array, targetSum):
    # Write your code here.
	solution = []
    for i in range(len(array) - 1):
		currentNum = array[i]
		for j in range(i + 1,len(array)):
			secondNum = array[j]
			if currentNum + secondNum == targetSum:
				if currentNum > secondNum:
					return [secondNum, currentNum]
				else:
					return [currentNum, secondNum]
	return []

Solution 2:
Hashing
time complexity : O(n)

def twoNumberSum(array, targetSum):
    nums = {
     }
	for num in array:
		if targetSum - num in nums:
			return [targetSum - num, num]
		else:
			nums[num] = True
	return []

Solution 3:
Sort
time complexity : O(nlogn)

def twoNumberSum(array, targetSum):
    array.sort()
	left = 0
	right = len(array) - 1
	while left < right:
		currentSum = array[left] + array[right]
		if currentSum == targetSum:
			return [array[left], array[right]]
		elif currentSum > targetSum:
			right -= 1
		else:
			left += 1
	return []

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