给定一个整数数列，找出其中和为特定值的那两个数。

你可以假设每个输入都只会有一种答案，同样的元素不能被重用。
示例:

给定 nums = [2, 7, 11, 15], target = 9

因为 nums[0] + nums[1] = 2 + 7 = 9
所以返回 [0, 1]

Java

class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] res = new int[2];
        for (int i = 0; i < nums.length - 1; i++) {
            for (int j = i + 1; j < nums.length; j++) {
                if (nums[i] + nums[j] == target) {
                    res[0] = i;
                    res[1] = j;
                    return res;
                }
            }
        }
        return res;
    }
}

优化

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
        int[] res = new int[2];
        for(int i = 0; i < nums.length; i++){
            int diff = target - nums[i];
            if(map.containsKey(nums[i])){
                res[0] = map.get(nums[i]);
                res[1] = i;
            }
            map.put(diff, i);
        }
        return res;
    }
}