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AlgorithmHomework1.md

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+# Algorithm Homework 1
+> 姓名：阙建明
+>
+> 学号：1901213139
+
+- ## 作业要求
+  3道上课讲的LeetCode题目1，69，70
+
+- ## Leetcode problem 1 (两数之和)
+  - 算法思路：<br/>
+    用字典记录数字，以便在找到成对的另一个数的时候，可以快速的检索到前一个数的索引
+  
+  - Leetcode提交结果
+    ![result](AlgorithmHomework1.assets/7222676-c485f42b177afce2.png)
+  - 代码：
+    ```python
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        myDict = {}
+        for index, num in enumerate(nums):
+            if num in myDict:
+                return [myDict[num], index]
+            else:
+                myDict[target - num] = index
+    ```
+
+- ## Leetcode problem 69 (x 的平方根)
+  - 算法思路：二分法
+  
+  - Leetcode提交结果
+    ![result](AlgorithmHomework1.assets/7222676-6a5936bab1f54a06.png)
+  - 代码：
+    ```python
+    def mySqrt(self, x: int) -> int:
+        if x in [1, 2, 3]:
+            return 1
+        left = 0
+        right = x // 2
+        while True:
+            mid = (left + right) // 2
+            if left + 1 >= right:
+                break
+            if mid < x / mid:  # mid to small
+                left = mid
+            else:
+                right = mid
+        return right if right * right <= x else left
+    ```
+
+- ## Leetcode problem 70 (爬楼梯)
+  - 算法思路：动态规划。由于当前状态只取决于前两个状态，所以dp table可以精简为只用两个数表示
+  
+  - Leetcode提交结果
+    ![result](AlgorithmHomework1.assets/7222676-dde7e98b91cb5454.png)
+  - 代码：
+    ```python
+    def climbStairs(self, n: int) -> int:
+        pre, cur = 0, 1
+        for i in range(1, n + 1):
+            pre, cur = cur, pre + cur
+        return cur
+    ```