中序遍历笔记

一、迭代遍历

        1.遍历顺序为：左子树——根节点——右子树

        2.创建Stack栈进行辅助计算\将根节点入栈、

        3.结束条件为栈为空并且当前节点为空

        4.循环入栈左子节点当前节点为空，

        5.集合存储出栈元素、

        6.将当前节点设置为右节点

/**
 * 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) {
        ArrayList<Integer> ans = new ArrayList<>();
        if(root==null){
            return ans;
        }
        Stack<TreeNode> stack= new Stack<>();
        stack.push(root);
        TreeNode t=stack.pop();
        while(t!=null||!stack.isEmpty()){
           while(t!=null){
                stack.push(t);
                t=t.left;
           }
           TreeNode arrt=stack.pop();
           ans.add(arrt.val);
           t=arrt.right;
        }
        return ans;
    }
}

二、递归解法：

        1.分解递归：左递归、根节点直接加入list、右递归

        

/**
 * 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) {
        ArrayList<Integer> ans = new ArrayList<>();
        f(root,ans);
        return ans;
    }
     public void f(TreeNode root,ArrayList<Integer> ans) {
        if(root==null){
            return;
        }
        f(root.left,ans);//左递归
        ans.add(root.val);//根加入
        f(root.right,ans);//右递归
     }
}