feat: add solutions to lc problem: No.1110

No.1110.Delete Nodes And Return Forest

Author: yanglbme

solution/1100-1199/1110.Delete Nodes And Return Forest/README.md

@@ -45,7 +45,16 @@
 
 <!-- 这里可写通用的实现逻辑 -->
 
-**方法一：后序遍历**
+**方法一：DFS**
+
+我们先用哈希表或者一个长度为 $1001$ 的数组 $s$ 记录所有需要删除的节点。
+
+接下来，设计一个函数 $dfs(root)$，它会返回以 $root$ 为根的子树中，删除所有需要删除的节点后的树的根节点。函数 $dfs(root)$ 的执行逻辑如下：
+
+-   如果 $root$ 为空，那么我们返回空；
+-   否则，我们递归执行 $dfs(root.left)$ 和 $dfs(root.right)$，并将返回值分别赋给 $root.left$ 和 $root.right$。如果 $root$ 不需要被删除，那么我们返回 $root$；如果 $root$ 需要被删除，那么我们分别判断 $root.left$ 和 $root.right$ 是否为空，如果它们不为空，那么我们将它们加入答案数组中；最后返回空。
+
+在主函数中，我们调用 $dfs(root)$，如果结果不为空，说明根节点不需要被删除，我们再将根节点加入答案数组中。最后返回答案数组即可。
 
 时间复杂度 $O(n)$，空间复杂度 $O(n)$。其中 $n$ 是树的节点数。
 
@@ -63,27 +72,25 @@
 #         self.left = left
 #         self.right = right
 class Solution:
-    def delNodes(self, root: TreeNode, to_delete: List[int]) -> List[TreeNode]:
-        def dfs(fa, root):
+    def delNodes(
+        self, root: Optional[TreeNode], to_delete: List[int]
+    ) -> List[TreeNode]:
+        def dfs(root: Optional[TreeNode]) -> Optional[TreeNode]:
             if root is None:
-                return
-            dfs(root, root.left)
-            dfs(root, root.right)
-            if root.val in s:
-                if fa and fa.left == root:
-                    fa.left = None
-                if fa and fa.right == root:
-                    fa.right = None
-                if root.left:
-                    ans.append(root.left)
-                if root.right:
-                    ans.append(root.right)
+                return None
+            root.left, root.right = dfs(root.left), dfs(root.right)
+            if root.val not in s:
+                return root
+            if root.left:
+                ans.append(root.left)
+            if root.right:
+                ans.append(root.right)
+            return None
 
         s = set(to_delete)
         ans = []
-        if root.val not in s:
+        if dfs(root):
             ans.append(root)
-        dfs(None, root)
         return ans
 ```
 
@@ -108,82 +115,35 @@ class Solution:
  * }
  */
 class Solution {
-    public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
-        boolean[] del = new boolean[1001];
-        for (int d : to_delete) {
-            del[d] = true;
-        }
-        List<TreeNode> res = new ArrayList<>();
-        dfs(root, true, del, res);
-        return res;
-    }
-
-    private TreeNode dfs(TreeNode root, boolean isRoot, boolean[] del, List<TreeNode> res) {
-        if (root == null) {
-            return null;
-        }
-        boolean flag = del[root.val];
-        if (!flag && isRoot) {
-            res.add(root);
-        }
-        root.left = dfs(root.left, flag, del, res);
-        root.right = dfs(root.right, flag, del, res);
-        return flag ? null : root;
-    }
-}
-```
-
-```java
-/**
- * Definition for a binary tree node.
- * public class TreeNode {
- *     int val;
- *     TreeNode left;
- *     TreeNode right;
- *     TreeNode() {}
- *     TreeNode(int val) { this.val = val; }
- *     TreeNode(int val, TreeNode left, TreeNode right) {
- *         this.val = val;
- *         this.left = left;
- *         this.right = right;
- *     }
- * }
- */
-class Solution {
+    private boolean[] s = new boolean[1001];
     private List<TreeNode> ans = new ArrayList<>();
-    private Set<Integer> s = new HashSet<>();
 
     public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
-        for (int v : to_delete) {
-            s.add(v);
+        for (int x : to_delete) {
+            s[x] = true;
         }
-        if (!s.contains(root.val)) {
+        if (dfs(root) != null) {
             ans.add(root);
         }
-        dfs(null, root);
         return ans;
     }
 
-    private void dfs(TreeNode fa, TreeNode root) {
+    private TreeNode dfs(TreeNode root) {
         if (root == null) {
-            return;
+            return null;
         }
-        dfs(root, root.left);
-        dfs(root, root.right);
-        if (s.contains(root.val)) {
-            if (fa != null && fa.left == root) {
-                fa.left = null;
-            }
-            if (fa != null && fa.right == root) {
-                fa.right = null;
-            }
-            if (root.left != null) {
-                ans.add(root.left);
-            }
-            if (root.right != null) {
-                ans.add(root.right);
-            }
+        root.left = dfs(root.left);
+        root.right = dfs(root.right);
+        if (!s[root.val]) {
+            return root;
+        }
+        if (root.left != null) {
+            ans.add(root.left);
         }
+        if (root.right != null) {
+            ans.add(root.right);
+        }
+        return null;
     }
 }
 ```
@@ -205,23 +165,33 @@ class Solution {
 class Solution {
 public:
     vector<TreeNode*> delNodes(TreeNode* root, vector<int>& to_delete) {
+        bool s[1001];
+        memset(s, 0, sizeof(s));
+        for (int x : to_delete) {
+            s[x] = true;
+        }
         vector<TreeNode*> ans;
-        unordered_set<int> s(to_delete.begin(), to_delete.end());
-        if (!s.count(root->val)) ans.push_back(root);
-        dfs(nullptr, root, s, ans);
-        return ans;
-    }
-
-    void dfs(TreeNode* fa, TreeNode* root, unordered_set<int>& s, vector<TreeNode*>& ans) {
-        if (!root) return;
-        dfs(root, root->left, s, ans);
-        dfs(root, root->right, s, ans);
-        if (s.count(root->val)) {
-            if (fa && fa->left == root) fa->left = nullptr;
-            if (fa && fa->right == root) fa->right = nullptr;
-            if (root->left) ans.push_back(root->left);
-            if (root->right) ans.push_back(root->right);
+        function<TreeNode*(TreeNode*)> dfs = [&](TreeNode* root) -> TreeNode* {
+            if (!root) {
+                return nullptr;
+            }
+            root->left = dfs(root->left);
+            root->right = dfs(root->right);
+            if (!s[root->val]) {
+                return root;
+            }
+            if (root->left) {
+                ans.push_back(root->left);
+            }
+            if (root->right) {
+                ans.push_back(root->right);
+            }
+            return nullptr;
+        };
+        if (dfs(root)) {
+            ans.push_back(root);
         }
+        return ans;
     }
 };
 ```
@@ -237,40 +207,83 @@ public:
  *     Right *TreeNode
  * }
  */
-func delNodes(root *TreeNode, to_delete []int) []*TreeNode {
-	s := map[int]bool{}
-	for _, v := range to_delete {
-		s[v] = true
+func delNodes(root *TreeNode, to_delete []int) (ans []*TreeNode) {
+	s := make([]bool, 1001)
+	for _, x := range to_delete {
+		s[x] = true
 	}
-	ans := []*TreeNode{}
-	if !s[root.Val] {
-		ans = append(ans, root)
-	}
-	var fa *TreeNode
-	var dfs func(fa, root *TreeNode)
-	dfs = func(fa, root *TreeNode) {
+	var dfs func(*TreeNode) *TreeNode
+	dfs = func(root *TreeNode) *TreeNode {
 		if root == nil {
-			return
+			return nil
+		}
+		root.Left = dfs(root.Left)
+		root.Right = dfs(root.Right)
+		if !s[root.Val] {
+			return root
 		}
-		dfs(root, root.Left)
-		dfs(root, root.Right)
-		if s[root.Val] {
-			if fa != nil && fa.Left == root {
-				fa.Left = nil
-			}
-			if fa != nil && fa.Right == root {
-				fa.Right = nil
-			}
-			if root.Left != nil {
-				ans = append(ans, root.Left)
-			}
-			if root.Right != nil {
-				ans = append(ans, root.Right)
-			}
+		if root.Left != nil {
+			ans = append(ans, root.Left)
 		}
+		if root.Right != nil {
+			ans = append(ans, root.Right)
+		}
+		return nil
+	}
+	if dfs(root) != nil {
+		ans = append(ans, root)
 	}
-	dfs(fa, root)
-	return ans
+	return
+}
+```
+
+### **TypeScript**
+
+```ts
+/**
+ * Definition for a binary tree node.
+ * class TreeNode {
+ *     val: number
+ *     left: TreeNode | null
+ *     right: TreeNode | null
+ *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
+ *         this.val = (val===undefined ? 0 : val)
+ *         this.left = (left===undefined ? null : left)
+ *         this.right = (right===undefined ? null : right)
+ *     }
+ * }
+ */
+
+function delNodes(
+    root: TreeNode | null,
+    to_delete: number[],
+): Array<TreeNode | null> {
+    const s: boolean[] = Array(1001).fill(false);
+    for (const x of to_delete) {
+        s[x] = true;
+    }
+    const ans: Array<TreeNode | null> = [];
+    const dfs = (root: TreeNode | null): TreeNode | null => {
+        if (!root) {
+            return null;
+        }
+        root.left = dfs(root.left);
+        root.right = dfs(root.right);
+        if (!s[root.val]) {
+            return root;
+        }
+        if (root.left) {
+            ans.push(root.left);
+        }
+        if (root.right) {
+            ans.push(root.right);
+        }
+        return null;
+    };
+    if (dfs(root)) {
+        ans.push(root);
+    }
+    return ans;
 }
 ```