two number sum

Refer to : https://www.algoexpert.io/questions/Two%20Number%20Sum

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Two number sum

1. problem statement.

给定一个不含重复数字的数组和一个目标值，返回一个长度为2的数组，这个数组包含的两个数的和要等于目标值，原数组里面的值不能重复使用，如果没有这样的两个值使得它们的和等于目标值，返回空数组。

2. brute force（O(n^2), O(1)）

def twoNumberSum(array, targetSum):
    for i in range(len(array)-1):
        for j in range(i+1, len(array)):
            if array[i] + array[j] == targetSum:
                return[array[i], array[j]]
    return[]

class Program {
  public static int[] twoNumberSum(int[] array, int targetSum) {
    
        for(int i = 0; i < array.length-1; i++){
            for(int j = i+1; j < array.length; j++){
                if(array[i] + array[j] == targetSum){
                    return new int[] {array[i], array[j]};
                }
            }
        }
    return new int[0];
  }
}

3. hash map(O(n), O(n))

def twoNumberSum(array, targetSum):
    nums = {}
    for num in array:
        b = targetSum - num
        if b in nums:
            return[b, num]
        else:
            nums[num] = True //mark the num as true.
    return[]

class Program {
  public static int[] twoNumberSum(int[] array, int targetSum) {
    HashSet<Integer> set = new HashSet<>();
        for(int i: array){
            int j = targetSum - i;
            if(set.contains(j)){
                return new int[] {i,j};
            }else{
                set.add(i);
            }
        }
    return new int[0];
  }
}

4. Sort && Pointers(O(nlogn), O(1))

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

class Program {
  public static int[] twoNumberSum(int[] array, int targetSum) {
    Arrays.sort(array);
        int left = 0, right = array.length-1;
        while(left < right){
            int currSum = array[left] + array[right];
            if(currSum == targetSum){
                return new int[] {array[left], array[right]};
            }else if(currSum > targetSum){
                right--;
            }else if(currSum < targetSum){
                left++;
            }
        }
    return new int[0];
  }
}