Dealing with binary trees represented as arrays and determining weather the left or right branch of the tree is larger. The size of each branch is the sum of the node values. (`-1` is considered a non-existent node)

binary-trees-as-arrays.php

<?php

function solution($arr)
{
    // If the array is empty or there is only 1 value
    if (count($arr) <= 1) {
        return '';
    }

    // A quick check as to not redefine the function
    if (!function_exists('innerSolution')) {
        function innerSolution($arr, $index)
        {
            if ($index - 1 < count($arr)) {
                if ($arr[$index - 1] === -1) {
                    return 0;
                }
                return $arr[$index - 1] +
                    innerSolution($arr, $index * 2) +
                    innerSolution($arr, $index * 2 + 1);
            }
            return 0;
        }
    }

    $left = innerSolution($arr, 2);
    $right = innerSolution($arr, 3);

    return ($left == $right) ? '' : ($left < $right ? 'Right' : 'Left');
}

// The test array and expected solutions
$testArray = [
  	// Array contains test array as well as expected solution
    [[3, 6, 9, -1, 10], 'Left'],
    [[1, 4, 100, 5], 'Right'],
    [[1, 10, 5, 1, 0, 6], ''],
    [[1], ''],
    [[], ''],
];

// Loop through the test array
$i = 0;
foreach ($testArray as $test) {
    $i++;
    // Determine our variables to use the array and the expected solution
    $arr = $test[0];
    $solution = $test[1];

    // Run the test
    $result = solution($arr);
  
    // Did the solution pass or fail
    $status = $result == $solution ? '✔️ PASS' : '❌ FAIL';
  
      // Some checking for purely test result purposes
    if (empty($solution)) $solution = "''";
  	if (empty($result)) $result = "''";
  
  	// Dump out some visual info on the tests 
    print("Test Case {$i}: {$status}\r\n");
    print("   Expected: {$solution}\r\n");
    print("   Result:   {$result}\r\n\r\n");
}

Expected output would look like this..

Test Case 1: ✔️ PASS
   Expected: Left
   Result:   Left

Test Case 2: ✔️ PASS
   Expected: Right
   Result:   Right

Test Case 3: ✔️ PASS
   Expected: ''
   Result:   ''

Test Case 4: ✔️ PASS
   Expected: ''
   Result:   ''

Test Case 5: ✔️ PASS
   Expected: ''
   Result:   ''

Thanks for sharing this @Braunson , much appreciated! Excellent use of recursion for forming the binary tree from one-dimensional array! I tried the same solution with JavaScript and it worked fine, sharing it here if its useful.

Creates a binary tree from one dimensional array and checks the largest branch from the root recursively