【算法】Leetcode算法题-求两数之和

    //求两数之和
    private static int[] twoSum(int[] arr, int target) {
        //暴力枚举法
        for (int i = 0; i < arr.length; i++) {
            for (int j = i+1; j < arr.length; j++) {
                if(arr[i]+arr[j] == target){
                    //返回两个变量的方法
                    return new int[]{i,j};
                }
            }
        }
        return new int[0];
    }

  //求两数之和
    private static int[] twoSumHash(int[] arr, int target) {
        //哈希法
        Map<Integer,Integer> map = new HashMap<>();
        for (int i = 0; i < arr.length; i++) {
            map.put(arr[i],i);
        }
        //找到下标
        for (int i = 0; i < arr.length; i++) {
            int key = target - arr[i];
            if(map.containsKey(key)){
                if (i!=map.get(key)){
                    return new int[]{i,map.get(key)};
                }
            }
        }
        return new int[]{};
    }

  public static void main(String[] args) {
        int[] arr = {3,4,7,6,9,1};
        //int [] indexArr = twoSumViolence(arr,12);
        int [] indexArr = twoSumHash(arr,12);
        System.out.println(Arrays.toString(indexArr));
    }