1. 两数之和

public int[] twoSum(int[] nums, int target) {
        HashMap<Integer,Integer> map = new HashMap<Integer,Integer>();
        for (int i=0;i<nums.length;i++){
            if(map.containsKey(target-nums[i])){
                return new int[]{map.get(target-nums[i]),i};
            }else {
                map.put(nums[i],i);
            }
        }
        return new int[]{0,0};
    }