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前言

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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

/*** 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

/*** 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

/*** 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;}
}