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].

用字典，存储当前值的下标，判断是否存在

target

-

nums

[

i

]

1. public class Solution {
2. public int[] TwoSum(int[] nums, int target) {
3. if (nums == null || nums.Length < 1) {
4. return null;
5. }
6. Dictionary<int, int> dict = new Dictionary<int, int>();
7. for (int i = 0; i < nums.Length; i++) {
8. if (dict.ContainsKey(target - nums[i])) {
9. int[] arr = {
   dict[target - nums[i]], i};
10. return arr;
11. } else {
12. dict[nums[i]] = i;
13. }
14. }
15. return null;
16. }
17. }