从 n 个数中取出两个数, 使其和为 a

采用穷举法的话, 其时间复杂度为 O((n^2)/2). 略去穷举法的实现.

采用如下实现, 其时间复杂度为 O(n), 空间复杂度增至 O(n).

public class Test {

    public static void main(String[] args) {
        int[] numberArr = {3, 5, 0, 35, 3, 25, 6, 3, 7, 6};
        int sum = 13;
        int[] result = find(numberArr, sum);
        System.out.println("满足条件的两个数的索引为: " + Arrays.toString(result));
    }

    public static int[] find(int[] numberArr, int sum) {
        Map<Integer, Integer> temp = new HashMap<>();

        for (int i = 0; i < numberArr.length; i++) {
            int number = numberArr[i];
            int target = sum - number;
            Integer targetIdx = temp.get(target);

            if (targetIdx != null) {
                return new int[]{targetIdx, i};
            } else {
                temp.put(number, i);
            }
        }

        return null;
    }
}