1. 两数之和

public static int[] twoSum(int[] nums, int target) {
        Map<Integer,Integer> map = new HashMap();
        int[] a = new int[2];
        for (int i = 0; i < nums.length; i++) {
            if (map.containsKey(target-nums[i])){
                a[0] = map.get(target-nums[i]);
                a[1] = i;
            }else {
                map.put(nums[i],i);
            }
        }
        return a;
    }