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文章目录
- 前言
- [一、力扣894. 所有可能的真二叉树](#一、力扣894. 所有可能的真二叉树)
- [二、力扣998. 最大二叉树 II](#二、力扣998. 最大二叉树 II)
- [三、力扣1110. 删点成林](#三、力扣1110. 删点成林)
前言
二叉树的递归分为「遍历」和「分解问题」两种思维模式,这道题需要用到「分解问题」的思维。
一、力扣894. 所有可能的真二叉树
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 {
List<TreeNode>[] memo;
public List<TreeNode> allPossibleFBT(int n) {
if(n % 2 == 0){
return new LinkedList<>();
}
memo = new LinkedList[n+1];
return build(n);
}
public List<TreeNode> build(int n){
List<TreeNode> res = new LinkedList<>();
if(n == 1){
res.add(new TreeNode(0));
return res;
}
if(memo[n] != null){
return memo[n];
}
for(int i = 1; i < n; i += 2){
int j = n -i -1;
List<TreeNode> l = build(i);
List<TreeNode> r = build(j);
for(TreeNode lt : l){
for(TreeNode lr : r){
TreeNode cur = new TreeNode(0);
cur.left = lt;
cur.right = lr;
res.add(cur);
}
}
}
memo[n] = res;
return res;
}
}
二、力扣998. 最大二叉树 II
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 {
public TreeNode insertIntoMaxTree(TreeNode root, int val) {
if(root == null){
return new TreeNode(val);
}
if(root.val < val){
TreeNode temp = root;
root = new TreeNode(val);
root.left = temp;
return root;
}else{
root.right = insertIntoMaxTree(root.right,val);
return root;
}
}
}
三、力扣1110. 删点成林
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 {
HashSet<Integer> delSet = new HashSet<>();
List<TreeNode> res = new LinkedList<>();
public List<TreeNode> delNodes(TreeNode root, int[] to_delete) {
if(root == null){
return new LinkedList<>();
}
for(int i : to_delete){
delSet.add(i);
}
fun(root,false);
return res;
}
public TreeNode fun(TreeNode root, boolean hisParent){
if(root == null){
return null;
}
boolean isdelete = delSet.contains(root.val);
if(!isdelete && !hisParent){
res.add(root);
}
root.left = fun(root.left,!isdelete);
root.right = fun(root.right,!isdelete);
return isdelete ? null:root;
}
}