leetcode binary-tree-inorder-traversal输出树的中序遍历

左根右
递归版本：

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    private ArrayList<Integer> list = new ArrayList<Integer>();
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        inorderTree(root);
        return list;
    }
    public void inorderTree(TreeNode root){
        if(root==null) return ;
        inorderTree(root.left);
        list.add(root.val);
        inorderTree(root.right);
        return ;
    }
}

非递归版本（使用栈）

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
import java.util.*;
public class Solution {
    public ArrayList<Integer> inorderTraversal(TreeNode root) {
        
        ArrayList<Integer> list = new ArrayList<Integer>();
        Stack<TreeNode> stack = new Stack<TreeNode>();
        if(root==null)return list;
        while(!stack.isEmpty()||root!=null){
            if(root!=null){
                stack.push(root);
                root = root.left;
            }
            else{
                root = stack.pop();
                list.add(root.val);
                root = root.right;
            }
        }
        return list;
    }
}