Given an array of integers, return indices of the two numbers such that they add up to a specific target.
You may assume that each input would have exactly one solution, and you may not use the same element twice.

Example

Given nums = [2, 7, 11, 15], target = 9,
Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

Solution

    public int[] twoSum(int[] nums, int target) {
        for (int i = 0; i < nums.length; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[i] + nums[j] == target) {
                    return new int[]{i, j};
                }
            }
        }
        return null;
    }

思考过程：

    public static void main(String[] args) {

        int[] ints = new int[]{2, 7, 11, 15};

        for (int i = 0; i < ints.length; i++) {
            for (int j = i + 1; j < ints.length; j++) {
                System.out.print(ints[i] + ints[j] + "  ");
            }
            System.out.println();
        }
    }

把所有可能的组合打印出来

9  13  17  
18  22  
26