leetcode : two sum 解题报告

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.

 

Example:Given nums = [2, 7, 11, 15], target = 9,

Because nums[0] + nums[1] = 2 + 7 = 9,
return [0, 1].

方法一：
two pointers


方法二： hashmap

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

 

 

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