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AlgorithmHomework4.md

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+# Algorithm Homework 4
+
+> 姓名：阙建明
+>
+> 学号：1901213139
+
+## 作业要求：
+
+Leetcode: 16，17，19
+
+## 题解：
+
+- ### Leetcode problem 16
+
+  >  [最接近的三数之和](https://leetcode-cn.com/problems/3sum-closest/)
+
+  - **算法思路：** 
+
+    - 首先对nums进行从小到达排序；
+    - 然后依次遍历0 ~ size - 2，固定第一个数i，对i + 1 ~ size - 1的数用双指针法处理；
+    - 通过忽略已处理过的数来实现第一个数选取时的剪枝操作
+
+  - **Leetcode提交结果：**
+
+    ![image-20200326200557425](AlgorithmHomework4.assets/image-20200326200557425.png)
+
+  - **代码：**
+
+    ```python
+    class Solution:
+        def threeSumClosest(self, nums: List[int], target: int) -> int:
+            if len(nums) < 3:
+                return 0
+            elif len(nums) == 3:
+                return nums[0] + nums[1] + nums[2]
+            nums = sorted(nums)  # 先排序
+            result, lastFirst = nums[0] + nums[1] + nums[2], nums[0] - 1
+            for i in range(0, len(nums) - 2):
+                if nums[i] == lastFirst:
+                    continue
+                left, right, lastFirst = i + 1, len(nums) - 1, nums[i]
+                while left < right:
+                    tmp = nums[i] + nums[left] + nums[right]
+                    if abs(target - tmp) < abs(target - result):
+                        result = tmp
+                    if tmp < target:
+                        left += 1
+                    elif tmp > target:
+                        right -= 1
+                    else:
+                        return result
+            return result
+    ```
+
+- ### Leetcode problem 17
+
+  > [电话号码的字母组合](https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number/)
+
+  - **算法思路：** 
+
+    - 采用回溯法递归遍历
+    
+  - **Leetcode提交结果：**
+
+    ![image-20200326210047592](AlgorithmHomework4.assets/image-20200326210047592.png)
+
+  - **代码：**
+
+    ```python
+    class Solution:
+        numMap = {'2': "abc", '3': "def", '4': "ghi", '5': "jkl", '6': "mno", '7': "pqrs", '8': "tuv", '9': "wxyz"}
+        result = []
+        def _letterCombinations(self, path, digits: str, index):
+            if index == len(digits) - 1:
+                for ch in self.numMap[digits[index]]:
+                    self.result.append(path + ch)
+                return
+            for ch in self.numMap[digits[index]]:
+                path = path + ch
+                self._letterCombinations(path, digits, index + 1)
+                path = path[:len(path) - 1]
+    
+        def letterCombinations(self, digits: str) -> List[str]:
+            if digits == "":
+                return []
+            self.result = []
+            self._letterCombinations("", digits, 0)
+            return self.result
+    ```
+
+- ### Leetcode problem 19
+
+  > [删除链表的倒数第N个节点](https://leetcode-cn.com/problems/remove-nth-node-from-end-of-list/)
+
+  - **算法思路：** 
+  
+  - 用快慢指针法
+    - 先让快指针走n步，然后让快慢指针同时右移，快指针为None时，慢指针指向的节点即为要删除的节点；
+  - 对要删除的节点是头结点的情况进行特判。
+  
+- **Leetcode提交结果：**
+  
+  ![image-20200326211143566](AlgorithmHomework4.assets/image-20200326211143566.png)
+  
+  - **代码：**
+  
+    ```python
+    class Solution:
+        def removeNthFromEnd(self, head: ListNode, n: int) -> ListNode:
+            fast, slow, pre = head, head, self
+            try:
+                for i in range(n):
+                    fast = fast.next
+            except BaseException:
+                return head
+            if fast is None:
+                return head.next
+            while fast is not None:
+                fast, slow, pre = fast.next, slow.next, slow
+          pre.next = slow.next
+            return head
+    ```
+  
+