LeetCode 二叉树的中序遍历（递归和非递归算法）

二叉树的中序遍历
给定一个二叉树，返回它的中序 遍历。
示例：

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]

非递归（思路更清晰）：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    
    public List<Integer> inorderTraversal(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        if(root == null) return list;
        TreeNode cur = root;
        //将currentNode移动到最左边
        while(cur != null){
            stack.push(cur);
            cur = cur.left;
        }
        while(!stack.isEmpty()){
            //根结点的访问条件是只要左子树访问完毕，就可以访问根结点（左节点为空或者左节点为上次访问节点）
            cur = stack.pop();
            list.add(cur.val);
            //切换到右子树
            cur = cur.right;
            //访问右子树的最左边
            while(cur != null){
                stack.push(cur);
                cur = cur.left;
            }
        }
        
        return list;
        
    }
   
}

非递归：

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

递归：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    ArrayList<Integer> list = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null) return new ArrayList<>();
        inorderTraversal(root.left);
        list.add(root.val);
        inorderTraversal(root.right);
        return list;
    }
   
}