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public class TwoSumDemo {

    /**
     * 通过双重循环遍历数组中所有元素的两两组合
     * 当出现符合的就放回两个元素的下标
     * @param nums
     * @param target
     * @return
     */
    public static int[] twoSum1(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (target - nums[i] == nums[j]) {
                    return new int[]{i, j};
                }
            }
        }
        return null;
    }

    public static void main(String[] args) {
        int[] nums = new int[]{2, 7, 11, 15};
        int target = 9;
        int[] myIndex = twoSum1(nums, target);
        assert myIndex != null;
        for (int el : myIndex) {
            System.out.println(el);
        }
    }
}

更优算法

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public static int[] twoSum2(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int partnerNumber = target - nums[i];
            if (map.containsKey(partnerNumber)) {
                return new int[]{map.get(partnerNumber), i};
            }
            map.put(nums[i], i);
        }
        return null;
    }