1. 两数之和

代码：

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