How to Fix Two Sum Sorted Errors

In this tutorial, you'll learn about How to Fix Two Sum Sorted Errors. We cover key concepts, practical examples, and best practices.

Fix two sum sorted errors when two-pointer doesn't use sorted property or pointers move wrong direction.

Quick Fix

Wrong

def pair(a,t):
    for i in range(len(a)):
        for j in range(i+1,len(a)):
            if a[i]+a[j]==t: return [i,j]
    return [-1,-1]

O(n^2) brute force. Doesn't use sorted array property.

Right

def pair(a,t):
    l=0; r=len(a)-1
    while l<r:
        s=a[l]+a[r]
        if s==t: return [l,r]
        elif s<t: l+=1
        else: r-=1
    return [-1,-1]

[2,3,4,7,11,15], target=7 -> [0,2] (2+4=7). O(n) time, O(1) space.

Prevention

Two pointers from both ends. If sum < target, move left right. If sum > target, move right left.

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