[Daily Coding Problem] 1 (LeetCode 1). Find if two numbers in an array add up to k
阿新 • • 發佈:2018-12-22
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?
經典2-sum問題的變形. 最直觀的做法是first pass, 用HashMap儲存每個數和其index的mapping關係, 然後second pass來尋找是否有滿足要求的。 需要注意的一點是,同一個位置的數字不能使用兩次.
1 public class Solution { 2 public int[] twoSum(int[] nums, int target) { 3 Map<Integer, Integer> hm = new HashMap<Integer, Integer>(); 4for(int i = 0; i < nums.length; i++) 5 { 6 hm.put(nums[i], i); 7 } 8 9 for(int i = 0; i < nums.length; i++) 10 { 11 if(hm.containsKey(target - nums[i]) && i != hm.get(target - nums[i])) 12 { 13 int[] r = new int[2]; 14 r[0] = i; 15 r[1] = hm.get(target - nums[i]); 16 return r; 17 } 18 } 19 return null; 20 } 21 }
1 pass solution 就是一邊掃描陣列, 一邊更新Map。 如果找到滿足要求的pair, 直接返回, 否則把當前的數插入到Map中.
1 public class Solution { 2 public int[] twoSum(int[] nums, int target) { 3 Map<Integer, Integer> map = new HashMap<Integer, Integer>(); 4 for(int i = 0; i < nums.length; i++) { 5 if(map.containsKey(target - nums[i])) { 6 int[] r = new int[2]; 7 r[0] = map.get(target - nums[i]); 8 r[1] = i; 9 return r; 10 } 11 else { 12 map.put(nums[i], i); 13 } 14 } 15 return null; 16 } 17 }