readme.md

Leetcode

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].

Thoughts:

Solution1:

方法1: 暴力, n^2 方法2: 遍历2次, 第一遍构建hashmap, 第二遍查询. 方法3: 遍历1次, 在遍历i的过程中, 不断的把之前的i放入hashmap, 然后求target. 代码如下:

public class Solution {

  public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> maps = new HashMap<>(nums.length - 1);

    maps.put(nums[0], 0);
    for (int i = 1; i < nums.length; i++) {
      int neededNumber = target - nums[i];
      if(maps.containsKey(neededNumber)){
        return new int[]{maps.get(neededNumber), i};
      }else {
        maps.put(nums[i], i);
      }
    }
    return null;
  }

  public static void main(String[] args) {
    int[] inputs = new int[] {2, 7, 11, 15};
    int[] result = new Solution().twoSum(inputs, 9);
    Logs.print(result);
  }
}