************二叉树的前序遍历：

. - 力扣（LeetCode）

递归

public class LeetCode144 {class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode() {}public TreeNode(int val) {this.val = val;}public TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}}public List<Integer> preorderTraversal(TreeNode root) {List<Integer> res=new LinkedList<>();preOrder(root,res);return res;}private void preOrder(TreeNode root, List<Integer> res) {if (root==null){return;}res.add(root.val);preOrder(root.left,res);preOrder(root.right,res);}}

迭代：

 

/*** 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 List<Integer> preorderTraversal(TreeNode root) {LinkedList<TreeNode> stack=new LinkedList<>();List<Integer> res=new LinkedList<>();if (root==null){return res;}stack.push(root);while (!stack.isEmpty()){TreeNode pop = stack.pop();res.add(pop.val);if (pop.right!=null) stack.push(pop.right);if (pop.left!=null) stack.push(pop.left);}return res;}
}

二叉树的后续遍历：

. - 力扣（LeetCode）

public class LeetCode145 {class TreeNode{int val;TreeNode left;TreeNode right;public TreeNode() {}public TreeNode(int val) {this.val = val;}public TreeNode(int val, TreeNode left, TreeNode right) {this.val = val;this.left = left;this.right = right;}}public List<Integer> postorderTraversal(TreeNode root) {List<Integer> list=new LinkedList<>();postOrder(root,list);return list;}private void postOrder(TreeNode root, List<Integer> list) {if (root==null){return;}if (root.left!=null){postOrder(root.left,list);}if (root.right!=null){postOrder(root.right,list);}list.add(root.val);}}

迭代：不太理解

/*** 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 List<Integer> postorderTraversal(TreeNode root) {LinkedList<TreeNode> stack=new LinkedList<>();List<Integer> res=new LinkedList<>();if (root==null){return res;}stack.push(root);while (!stack.isEmpty()){TreeNode pop = stack.pop();res.add(pop.val);if (pop.left!=null) stack.push(pop.left);if (pop.right!=null) stack.push(pop.right);}Collections.reverse(res);return res;}
}

二叉树的中序遍历

. - 力扣（LeetCode）

/*** 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 List<Integer> inorderTraversal(TreeNode root) {List<Integer> res=new LinkedList<>();LinkedList<TreeNode> stack=new LinkedList<>();TreeNode cur=root;while (cur!=null||!stack.isEmpty()){if (cur!=null){stack.push(cur);cur=cur.left;}else{cur = stack.pop();res.add(cur.val);cur=cur.right;}}return res;}
}

****二叉树的最小深度：
 

. - 力扣（LeetCode）

 

/*** 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 int minDepth(TreeNode root) {//只有当左右孩子都为null时才算if (root==null) return 0;if (root.left==null&&root.right==null) return 1;int min=Integer.MAX_VALUE;if (root.left!=null) min=Math.min(min,minDepth(root.left));if (root.right!=null) min=Math.min(min,minDepth(root.right));return 1+min;}
}

翻转二叉树：

. - 力扣（LeetCode）

递归：可以使用递归的大多都可以使用栈进行迭代

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;}}public TreeNode invertTree(TreeNode root) {if (root==null) return root;if (root.left==null&&root.right==null) return root;TreeNode temp=root.left;root.left=root.right;root.right=temp;invertTree(root.left);invertTree(root.right);return root;}

迭代：

public TreeNode invertTree2(TreeNode root) {if (root==null) return root;if (root.left==null&&root.right==null) return root;LinkedList<TreeNode> stack=new LinkedList<>();stack.push(root);while (!stack.isEmpty()){TreeNode pop = stack.pop();TreeNode temp=pop.left;pop.left=pop.right;pop.right=temp;if (pop.left!=null) stack.push(pop.left);if (pop.right!=null) stack.push(pop.right);}return root;}