1.Two Sum

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].
算法复杂度O(nlogn)

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

discuss里中O(n)方法

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