third week

CodePythonFollow · CodePythonFollow · commit 82c7a2c2e6aa · 2020-12-20T21:19:00.000+08:00
diff --git a/Week_03/README.md b/Week_03/README.md
@@ -1 +1,49 @@
-学习笔记
+# 作业
+
++ [二叉树的最近公共祖先](https://leetcode-cn.com/problems/lowest-common-ancestor-of-a-binary-tree/)
+
+  ```c++
+  class Solution {
+  public:
+      TreeNode* ans;
+      bool dfs(TreeNode* root, TreeNode* p, TreeNode* q) {
+          if (root == nullptr) return false;
+          bool lson = dfs(root->left, p, q);
+          bool rson = dfs(root->right, p, q);
+          if ((lson && rson) || ((root->val == p->val || root->val == q->val) && (lson || rson))) {
+              ans = root;
+          } 
+          return lson || rson || (root->val == p->val || root->val == q->val);
+      }
+      TreeNode* lowestCommonAncestor(TreeNode* root, TreeNode* p, TreeNode* q) {
+          dfs(root, p, q);
+          return ans;
+      }
+  };
+  ```
+
++ [组合](https://leetcode-cn.com/problems/combinations/)
+
+  ```c++
+  class Solution:
+      def combine(self, n: int, k: int) -> List[List[int]]:
+          if n < k or n < 1:
+              return []
+          if k == 0:
+              return [[]]
+          if n == k:
+              return [[i for i in range(1, n+1)]]
+          ans1 = self.combine(n-1, k-1)
+          ans2 = self.combine(n-1, k)
+          print(n, k, ans1, ans2)
+          if ans1:
+              for i in ans1:
+                  i.append(n)
+          return ans1+ans2
+  
+  ```
+
+# 总结
+
++ 本结知识确实比较薄弱，还需要多刷题，多理解。
++ 记住模板之后再刷题效果确实会好很多。
