Solution to Booj codetest

EquilibriumIndex.php

@@ -1,9 +1,46 @@
 <?php
+/*
+ * Drew Henry (drewhenr@usc.edu)
+ * 
+ * To verify an equilibrium index we will need to sum the entire
+ * array at least once so start by doing this and storing the result
+ * in a variable. Once we have the initial sum we need only perform
+ * two addition operations and one comparison operation per index
+ * giving us a total O(n) operations. By placing the sum comparison
+ * between the sum updates we avoid the need for extra storage
+ * variables and require only a simple foreach loop over the array.
+ * 
+ * Note that this code does not attempt to validate input and will
+ * work without throwing any errors as long as an array is given
+ * with numeric or numeric convetable (e.g. true, null, '0', etc.)
+ * values.
+ */
 
-# Put a valid docblock here!
-function getEquilibriums($arr) {
-	$output = array();
-	# Logic goes here!
+/**
+ * Finds and returns the equilibrium indices of the provided array.
+ *
+ * @param int[] $arr Array of integers to be checked.
+ *
+ * @return int[] Array of equilibrium indices.
+ */
+function getEquilibriums(array $arr) {
+	$output = [];
+
+	# Set the initial sums
+	$sum_l = 0;
+	$sum_r = array_sum($arr);
+
+	# Iterate over all indices
+	foreach($arr as $k=>$v){
+		# Remove the current element from the right sum
+		$sum_r -= $v;
+		# Check if left sum equals right sum and update output array
+		if($sum_l === $sum_r){
+			$output[] = $k;
+		}
+		# Add the current element to the left sum
+		$sum_l += $v;
+	}
+
 	return $output;
 }
-