drafting some posts

source/_drafts/leetcode-1-two-sum-md.md

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+---
+title: leetcode-1-two-sum
+tags: ["leetcode", "solution"]
+date: 2019-11-24 21:06:37
+---
+
+> 嗯，虽然以前就就在Leetcode上面刷过题，但是都没有坚持下去。这次准备重操旧业，希望能起码做到一天一道题吧。
+
+首先是A+B问题，按理说是所有题目的开始。但是很遗憾的是我还是提交了三次才AC，感觉自己弱爆了。废话不多说，进入正题。
+
+<!--more-->
+
+```
+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, 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].
+```
+
+题意很简单，就是给一个list，然后给定一个target，需要从list中找到两个数的和是target，然后输出这两个数的下标。需要注意的是每个元素只能用一次，另外输出的list中元素的顺序不影响结果。
+
+我很快就完成了，但是第一版代码是错的，代码如：
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        m = dict()
+        for i in range(len(nums)):
+            if i not in m:
+                m[nums[i]] = i
+            if nums[i] in m and target-nums[i] in m:
+                return [m[nums[i]], m[target - nums[i]]]
+        return []
+```
+
+致命的问题在于如果test case是[3,2,1] 6，那么输出是 [0,0]，也就是说第一个元素用了两次，嗯，这么简单的边界都翻车。
+
+然后很快又改好了，加了判断条件，代码如：
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        m = dict()
+        for i in range(len(nums)):
+            if nums[i] not in m:
+                m[nums[i]] = i
+            if nums[i] in m and target-nums[i] in m and m[nums[i]] != m[target - nums[i]]:
+                return [m[nums[i]], m[target - nums[i]]]
+        return []
+```
+
+然后又WA了。。。嗯，仔细分析了一下，发现test case是[3,3] 6的时候，输出是空。嗯，分支条件写错了。不应该判断两个数字都在字典里面，因为如果有两个相同的元素的话只有一个会加入到字典中。为了解决这个问题，嗯，直接换个写法就好，如：
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        m = dict()
+        for i in range(len(nums)):
+            if nums[i] not in m:
+                m[nums[i]] = i
+            if target-nums[i] in m and i != m[target - nums[i]]:
+                return [i, m[target - nums[i]]]
+```
+
+嗯，总算AC了。我感觉每天一题的目标估计是要GG了。
+
+稍微看了下题解和别人的代码，我发现这么写更加优雅：
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        m = dict()
+        for i, n in enumerate(nums):
+            if target-n in m:
+                return [m[target - n], i]
+            if n not in m:
+                m[n] = i
+```
+
+# Refer
+
+[1. Two Sum](https://leetcode.com/problems/two-sum/)
+
+[1. Two Sum](https://leetcode.com/articles/two-sum/)
+# Appendix