1. Two Sum

Given an array of integers, return indices of the two numbers suchthat they add up to a specific target.You may assume that each input would have exactly one solution, andyou 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].1.暴力 O(n^2)2.排序后双指针 O(nlogn)3.hash o(n)

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