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.

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

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

UPDATE (2016/2/13):

The return format had been changed to zero-based indices. Please read the above updated description carefully.

This question has a small trap should pay attention, say the following solution

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

for this specific case:

[0,4,3,0], 0, ===> the above solution gives [3,3], not [0,3]. to fix this bug.

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> map = new HashMap<>();
        int[] res = new int[2];
        for(int i=0; i< nums.length; i++){
            if(map.get(nums[i]) != null){
                res[0] = map.get(nums[i]);
                res[1] = i;
                return res;
            }else{
                map.put(target-nums[i], i); // cross reference the key and pos.
            }

        }
        return res;
    }
}