暴力：双循环

正解：

哈希表<值，下标>

先find找不到再insert，保证不会和自己匹配。

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

链接：https://leetcode.cn/problems/two-sum/solutions/434597/liang-shu-zhi-he-by-leetcode-solution/