leetcode有相关的题目

• 二叉树的前序遍历
• 二叉树的中序遍历
• 二叉树的后序遍历

转一个我觉得写的很清楚的题解：
https://leetcode-cn.com/problems/binary-tree-preorder-traversal/solution/leetcodesuan-fa-xiu-lian-dong-hua-yan-shi-xbian-2/

import java.util.ArrayList;
import java.util.List;
import java.util.Stack;


public class Search {
    private List<Integer> list;   //存储最后的结果
//前序 递归算法
    public List<Integer> preorderTraversal1(TreeNode root) {
        list = new ArrayList<>();
        preVisit(root);
        return list;
    }
    public void preVisit(TreeNode root){
        if(root == null) return;
        else{
            list.add(root.val);
            preVisit(root.left);
            preVisit(root.right);
        }
    }
//前序  迭代算法
    public List<Integer> preorderTraversal(TreeNode root) {

        List<Integer> res = new ArrayList<>();
        if(root==null) return res;
        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);
        while (!stack.empty()){
            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;
    }

//中序  递归算法

    public List<Integer> inorderTraversal1(TreeNode root) {
        list = new ArrayList<>();
        inorderVisit(root);
        return list;
    }

    public void inorderVisit(TreeNode root){
        if(root == null) return;
        else{
            inorderVisit(root.left);
            list.add(root.val);
            inorderVisit(root.right);

        }
    }

//中序  迭代算法
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode cur = root;
        while(cur!=null || !stack.empty()){
            while (cur!=null){
                stack.push(cur);
                cur = cur.left;
            }
            TreeNode pop = stack.pop();
            res.add(pop.val);
            if(pop.right!=null){
                cur=pop.right;
            }
        }
        return res;
    }

//后序   递归算法
    public List<Integer> postorderTraversal1(TreeNode root) {
        list = new ArrayList<>();
        postVisit(root);
        return list;
    }
    public void postVisit(TreeNode root){
        if(root == null) return;
        else{
            postVisit(root.left);
            postVisit(root.right);
            list.add(root.val);
        }
    }
//后序  迭代算法
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> res  = new ArrayList<>();
        if(root == null )return res;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode reNode = root;
        stack.push(root);
        while(!stack.empty()){
            TreeNode cur = stack.peek();
            if(cur.left!=reNode && cur.right!=reNode && cur.left!=null){
                stack.push(cur.left);
            }
            else if(cur.right!=reNode && cur.right!=null){
                stack.push(cur.right);
            }
            else{
                res.add(cur.val);
                stack.pop();
                reNode=cur;
            }
        }
        return res;
    }



}