Write a method int zeroSum(int[] a) that checks if array a contains two distinct numbers that sum to zero. If so, return the index of the first number. If not, return -1. The expected runtime must be O(n) where n is the array length. Use a HashMap (the key is the element, the value is its index). For each element of the array, check if it's negation is a key in the map. If it is, return its index. Then add the pair (element, index) to the map.