Grokking the Coding Interview: Patterns for Coding Questions
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Pair with Target Sum (easy)

Problem Statement

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Problem Statement

Given an array of numbers sorted in ascending order and a target sum, find a pair in the array whose sum is equal to the given target.

Write a function to return the indices of the two numbers (i.e. the pair) such that they add up to the given target. If no such pair exists return [-1, -1].

Example 1:

Input: [1, 2, 3, 4, 6], target=6
Output: [1, 3]
Explanation: The numbers at index 1 and 3 add up to 6: 2+4=6

Example 2:

Input: [2, 5, 9, 11], target=11
Output: [0, 2]
Explanation: The numbers at index 0 and 2 add up to 11: 2+9=11

Constraints:

  • 2 <= arr.length <= 10<sup>4</sup>
  • -10<sup>9</sup> <= arr[i] <= 10<sup>9</sup>
  • -10<sup>9</sup> <= target <= 10<sup>9</sup>
  • Only one valid answer exists.

Try it yourself

Try solving this question here:

Python3
Python3
. . . .
A

Anonymous

· 3 years ago

There's a test case in the problem that violates the assumption that only a single valid answer exists.

Example:

arr = [2,30,33,64,67,98]

target_sum = 100

This scenario has two valid answers: [0, 5] and [2, 4].

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A

Anonymous

· 3 years ago

class Solution:
  def search(self, arr, target_sum):
    # TODO: Write your code here
    l, r = 0, len(arr) - 1

    while l<r:
      curr= arr[l] + arr[r]
      if curr>target_sum:
        r-=1
      elif curr<target_sum:
        l+=1
      else:
        return [l,r]
    return [-1, -1]
A

Anonymous

· 3 years ago

Question text should say to return -1,-1 when no result exists.

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A

Anonymous

· 4 years ago

FYI if anyone else thinks they're going crazy two-pointer and sliding window have switched spots. I think it's a correct move as sliding window feels like a more specialized version of two-pointer so you get the benefit of general to specifics.

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A

Anonymous

· 4 years ago

I think it's important to mention that the solution with a HashMap works even when the array is unsorted.

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A

Anonymous

· 4 years ago

I keep getting [-1,-1]as the result even though its clear that there is a correct answer from the output

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A

Anonymous

· 5 years ago

This one is NOT working on LeetCode. Can someone help me with this please?

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A

Anonymous

· 5 years ago

Anyone can explain why the return is [-1,-1] here?

function pair_with_target_sum(arr, targetSum) { const nums = {}; // to store numbers and their indices for (let i = 0; i < arr.length; i++) { const num = arr[i]; if (targetSum - num in nums) { return [nums[targetSum - num], i]; } nums[arr[i]] = i; } return [-1, -1]; }

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A

Anonymous

· 5 years ago

The first solution for "Pair with Target Sum. I guess it is not working for the inputs. [3,2,4] and target as 6;

pair_with_target([3,2,4], 6)

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Problem Statement

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