[LeetCode] 1. Two Sum 两数和
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, and you may not use the same element twice.
Example:
Given nums = [2, 7, 11, 15], target = 9, Because nums[0] + nums[1] = 2 + 7 = 9, return [0, 1].
给一个没排序的包含整数的数组，找出使得两个数的和是给定值的indices。假设每个输入只有一个答案， 同一元素不会用到两次。
解法：
1. 暴力解法，两个for循环遍历，两个数相加和目标数比较。Time: O(n^2)
2. 先对数组快速排序，然后用两个指针分别指向头和尾，每次比较头尾两个数的和，如果比target小，头标记右移，如果大，尾标记左移。需要注意记录快速排序前后数字的位置变化。Time: O(n)
3. 先遍历一遍数组，建立数字和index的HashMap，然后再遍历一遍，开始查找target - num[i]是否在map中，如果在，找到并返回index。Time: O(n) Space: O(n)
Java：解法1
public static int[] twoSum(int[] numbers, int target) {
int[] ret = new int[2];
for (int i = 0; i < numbers.length; i++) {
for (int j = i + 1; j < numbers.length; j++) {
if (numbers[i] + numbers[j] == target) {
ret[0] = i + 1;
ret[1] = j + 1;
}
}
}
return ret;
}
Java：解法2
class Pair implements Comparable<Pair>{
public int number;
public int idx;
public Pair(int number, int idx){
this.number = number;
this.idx = idx;
}
public int compareTo(Pair other){
return this.number - other.number;
}
}
public class Solution {
public int[] twoSum(int[] numbers, int target) {
int n = numbers.length;
Pair[] pairs = new Pair[n];
for(int i = 0; i < n; ++i){
pairs[i] = new Pair(numbers[i], i + 1);
}
Arrays.sort(pairs);
int [] result = new int[2];
int begin = 0;
int end = n - 1;
while(begin < end){
if(pairs[begin].number + pairs[end].number < target){
begin++;
}
else if (pairs[begin].number + pairs[end].number > target){
end--;
}
else{
if(pairs[begin].idx > pairs[end].idx){
result[0] = pairs[end].idx;
result[1] = pairs[begin].idx;
}else{
result[0] = pairs[begin].idx;
result[1] = pairs[end].idx;
}
break;
}
}
return result;
}
}
Java: 解法3，Two loops
public class Solution {
public int[] twoSum(int[] nums, int target) {
HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();
int[] res = new int[2];
for (int i = 0; i < nums.length; ++i) {
m.put(nums[i], i);
}
for (int i = 0; i < nums.length; ++i) {
int t = target - nums[i];
if (m.containsKey(t) && m.get(t) != i) {
res[0] = i;
res[1] = m.get(t);
break;
}
}
return res;
}
}
Java: 解法3，one loop
public class Solution {
public int[] twoSum(int[] nums, int target) {
HashMap<Integer, Integer> m = new HashMap<Integer, Integer>();
int[] res = new int[2];
for (int i = 0; i < nums.length; ++i) {
if (m.containsKey(target - nums[i])) {
res[0] = i;
res[1] = m.get(target - nums[i]);
break;
}
m.put(nums[i], i);
}
return res;
}
}
Python:
class Solution(object):
def twoSum(self, nums, target):
hash_map = {}
for i, v in enumerate(nums):
hash_map[v] = i
for index1, value in enumerate(nums):
if target - value in hash_map:
index2 = hash_map[target - value]
if index1 != index2:
return [index1, index2]
Python:
class Solution(object):
def twoSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[int]
"""
lookup = {}
for i, num in enumerate(nums):
if target - num in lookup:
return [lookup[target - num], i]
lookup[num] = i
Python: wo
class Solution(object):
def twoSum(self, nums, target):
"""
:type nums: List[int]
:type target: int
:rtype: List[int]
"""
lookup = {}
for i in range(len(nums)):
if target - nums[i] in lookup:
return [lookup[target - nums[i]], i]
lookup[nums[i]] = i
Python:
class Solution(object):
def twoSum(self, nums, target):
if len(nums) <= 1:
return False
buff_dict = {}
for i in range(len(nums)):
if nums[i] in buff_dict:
return [buff_dict[nums[i]], i]
else:
buff_dict[target - nums[i]] = i
C++:
class Solution {
public:
vector<int> twoSum(vector<int>& nums, int target) {
unordered_map<int, int> lookup;
for (int i = 0; i < nums.size(); ++i) {
if (lookup.count(target - nums[i])) {
return {lookup[target - nums[i]], i};
}
lookup[nums[i]] = i;
}
return {};
}
};
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