1.找两个数下标Two Sum

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]

public class Solution {
    public int[] TwoSum(int[] nums, int target) {
		if (nums == null || nums.Length < 1) {
			return null;
		}
		Dictionary<int, int> dict = new Dictionary<int, int>();
		for (int i = 0; i < nums.Length; i++) {
			if (dict.ContainsKey(target - nums[i])) {
				int[] arr = { dict[target - nums[i]], i};
				return arr;
			} else {
				dict[nums[i]] = i;
			}
		}
		return null;
    }
}

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