[leetcode][1]two sum

Problem

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].

Solution

在一个数组中找到不同下标的2个数使得和为某个值，假设数组长度为m。最简单的做法是两层循环，时间复杂度O(m2)。如果用空间换时间，用一个HashMap 存储已经遍历过的数组元素及对应下标，如果在遍历target - nums[i] 时发现HashMap 中已经存在该值，则可以得到结果。时间复杂度为O(m)。

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