Leetcode-1. Two Sum

题目

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].

代码

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

总结

• 使用HashMap数据结构辅助解题；
• 只要Hash算法够好，对Map的操作能够做到O(1)的操作；
• 有个小技巧先确定result[1]的值，再确定result[0]的值，从后向前匹配，这样能够找到同一个值在数组中出现两次的情况。