[Daily Coding Problem] 1 (LeetCode 1). Find if two numbers in an array add up to k

This problem was recently asked by Google.

Given a list of numbers and a number k, return whether any two numbers from the list add up to k.

For example, given [10, 15, 3, 7] and k of 17, return true since 10 + 7 is 17.

Bonus: Can you do this in one pass?

经典2-sum问题的变形.  最直观的做法是first pass, 用HashMap保存每个数和其index的mapping关系， 然后second pass来寻找是否有满足要求的。 需要注意的一点是，同一个位置的数字不能使用两次.

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

1 pass solution 就是一边扫描数组， 一边更新Map。 如果找到满足要求的pair， 直接返回， 否则把当前的数插入到Map中.

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