1. Two Sum

1. Two Sum

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Question Editorial Solution

Total Accepted: 210007  Total Submissions: 937405  Difficulty: Easy

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.

74ms：

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

5ms：

public class Solution {
    public int[] twoSum(int[] nums, int target) {
        int[] nums_sorted=new int[nums.length];
        System.arraycopy(nums,0,nums_sorted,0,nums.length);
        //Quicksort.
        Arrays.sort(nums_sorted);

        //Find the two numbers. O(n)
        int start=0;
        int end=nums_sorted.length;
        while(start<end){
            while(nums_sorted[start]+nums_sorted[--end]>target);
            if(nums_sorted[end]+nums_sorted[start]==target)
                break;
            while(nums_sorted[++start]+nums_sorted[end]<target);
            if(nums_sorted[end]+nums_sorted[start]==target)
                break;
        }

        //find the indices of the two numbers
        int[] ret=new int[2];
        int index=0;
        int a=nums_sorted[start];
        int b=nums_sorted[end];
        for(int i=0;i<nums.length;i++)
            if(nums[i]==a || nums[i]==b)
                ret[index++]=i;
        return ret;
    }
}

8ms：利用hash表

public class Solution {
public static void main(String[] args) {
		int[] a = { 2, 7, 11, 15 };
		System.out.println(twoSum(a, 9));
	}
	
	public static int[] twoSum(int[] nums, int target) {
        int[] result = new int[2];
        int n2;
        Integer idx;
        Map<Integer, Integer> map = new HashMap<Integer, Integer>(nums.length);
        
        for (int i = 0; i < nums.length; i++) {
			map.put(nums[i], i);
		}
        for (int i = 0; i < nums.length - 1; i++) {
			n2 = target - nums[i];
			idx = map.get(n2);
			if(idx != null && idx > i){
				result[0] = i;
				result[1] = idx;
				break;
			}
		}
        return result;
    }
}