1.理论基础

1. 二叉树定义

   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;}
   }
   

2. 二叉树种类
   在我们解题过程中二叉树有两种主要的形式：满二叉树和完全二叉树。
   满二叉树: 只有度为0和度为2的节点；
   完全二叉树：除了最底层可能没填满，其他每层的节点数都是最大值，且最底层的节点都集中在该层的最左边；

3. 二叉树存储方式
   主要为链表方式和数组方式；
   

4. 二叉树的遍历方式
   深度优先遍历：下面的前中后指的是中间节点的遍历位置

   • 前序遍历（中左右）
     递归法：

     class Solution {public List preorderTraversal(TreeNode root) {List result = new ArrayList();preorder(root, result);return result;}public void preorder(TreeNode root, List result) {// 迭代终止条件if (root == null) {return;}// 中左右result.add(root.val);preorder(root.left, result);preorder(root.right, result);}
     }
     

     迭代法：

     class Solution {public List preorderTraversal(TreeNode root) {List result = new ArrayList<>();if (root == null){return result;}Stack stack = new Stack<>();stack.push(root);while (!stack.isEmpty()){TreeNode node = stack.pop();result.add(node.val);if (node.right != null){stack.push(node.right);}if (node.left != null){stack.push(node.left);}}return result;}
     }
     

   • 中序遍历（左中右）
     递归法:

     // 中序遍历·递归·LC94_二叉树的中序遍历
     class Solution {public List inorderTraversal(TreeNode root) {List res = new ArrayList<>();inorder(root, res);return res;}void inorder(TreeNode root, List list) {if (root == null) {return;}// 左中右inorder(root.left, list);list.add(root.val);             inorder(root.right, list);}
     }
     

     迭代法：

     class Solution {public List inorderTraversal(TreeNode root) {List result = new ArrayList<>();if (root == null){return result;}Stack stack = new Stack<>();TreeNode cur = root;while (cur != null || !stack.isEmpty()){if (cur != null){stack.push(cur);cur = cur.left;}else{cur = stack.pop();result.add(cur.val);cur = cur.right;}}return result;}
     }
     

   • 后序遍历（左右中 ）
     递归法：

     class Solution {public List postorderTraversal(TreeNode root) {List res = new ArrayList<>();postorder(root, res);return res;}void postorder(TreeNode root, List list) {if (root == null) {return;}// 左右中postorder(root.left, list);postorder(root.right, list);list.add(root.val);             }
     }
     

     迭代法：

     class Solution {public List postorderTraversal(TreeNode root) {List result = new ArrayList<>();if (root == null){return result;}Stack stack = new Stack<>();stack.push(root);while (!stack.isEmpty()){TreeNode node = stack.pop();result.add(node.val);if (node.left != null){stack.push(node.left);}if (node.right != null){stack.push(node.right);}}Collections.reverse(result);return result;}
     }
     

   广度优先遍历

   • 层次遍历