O(n)

[JuliyaShi]

var twoSum = function(nums, target) {
let map = new Map();
for(i = 0; i<nums.length; i ++){
let num1 = nums[i];
let num2 = target - num1;
if(map.has(num2)){
return [i, map.get(num2)]
}
map.set(num1, i)
}
};