Leetcode刷題java之二叉樹的前序中序後續遍歷非遞歸實現（一天一道編程題之二十四天）

前序遍歷

/**
 * 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> preorderTraversal(TreeNode root) {
        TreeNode cur=root;
        Stack<TreeNode> stack=new Stack<>();
        List<Integer> result=new ArrayList<>();
        while(!stack.isEmpty()||cur!=null)
        {
            while(cur!=null)
            {
                result.add(cur.val);
                stack.push(cur);
                cur=cur.left;
            }
            //取出一個進行回溯
            cur=stack.pop();
            cur=cur.right;
        }
        return result;
    }
}

中序遍歷

/**
 * 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) {
        TreeNode cur=root;
        Stack<TreeNode> stack=new Stack<>();
        List<Integer> result=new ArrayList<>();
        while(!stack.isEmpty()||cur!=null)
        {
            while(cur!=null)
            {
                stack.push(cur);
                cur=cur.left;
            }
            cur=stack.pop();
            //從棧中取出一個對右進行回溯
            result.add(cur.val);
            cur=cur.right;
        }
        return result;
    }
}

後序遍歷：

/**
 * 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> postorderTraversal(TreeNode root) {
        TreeNode cur=root;
        TreeNode last=null;
        Stack<TreeNode> stack=new Stack<>();
        List<Integer> result=new ArrayList<>();
        while(!stack.isEmpty()||cur!=null)
        {
            while(cur!=null)
            {
                stack.push(cur);
                cur=cur.left;
            }
            TreeNode top=stack.peek();
            if(top.right==null||top.right==last)
            {
                result.add(top.val);
                stack.pop();
                last=top;
            }else
            {
                cur=top.right;
            }
        }
        return result;
    }
}