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.

 

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public class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> numToIndex = new HashMap<Integer,Integer>();
        for(int i = 0; i<nums.length; ++i)
        {
            numToIndex.put(nums[i], i);
        }
        for(int i = 0; i<nums.length; ++i)
        {
            Integer otherIndex = numToIndex.get(target-nums[i]);
            if(otherIndex!=null && i != otherIndex)
                return new int[]{i, otherIndex};
        }
        return new int[2];
    }
}