Third test case PHP for loop does not increment

Question

Given an array of integers nums and an integer target, return indices of the two numbers such that they add up to target. I've tried a few modified versions of the solution below.

function twoSum($nums, $target) {
    $arr = array();
    for($i = 0; $i < count($nums); $i++){
        
        if($nums[$i] + $nums[$i+1] == $target){
            array_push($arr, array_search($nums[$i], $nums), array_search($nums[++$i], $nums));
            break;
        }
    }
    return $arr;
}

When passed three test cases the first two return the correct outcome the last case returns the first correct index but the last index is (technically) incorrect.

Test cases used:

1. nums = [2,7,11,15], target = 9
   expected output: [0,1]
   actual output: [0,1]

2. nums = [3,2,4], target = 6
   expected output: [1,2]
   actual output: [1,2]

3. nums = [3,3], target = 6
   expected output: [0,1]
   actual output: [0,0]

Answer 1

Your approach is incorrect because you assume 2 consecutive indices can lead to a target value in this line, which is incorrect. The pair could be any combination.

$nums[$i] + $nums[$i+1] == $target

It is also not considering index out of bounds exception for $i + 1. Rest of the code would surely go in incorrect direction anyway.

---

The steps to solve this is pretty straightforward.

• Use a simple associative array, say $set which will store key as the array element and value as the index of that element in the array.

• Now, when looping over the array, if target - current_element key(the other number) exists in the $set, you got the pair.

Snippet:

<?php

function twoSum($nums, $target) {
    $set = [];
    foreach($nums as $index => $curr_element){
        $cousin_value = $target - $curr_element;
        if(isset( $set[ $cousin_value ] )){
            return [  $set[ $cousin_value ], $index ];
        }
        $set[ $curr_element ] = $index;
    }
    return -1;
}

print_r(twoSum([3,3],6));