二叉樹中序遍歷

原題

　　Given a binary tree, return the inorder traversal of its nodes’ values.

題目大意

　　對一棵二叉樹進行中序遍歷。

解題思路

　　解法一：遞歸實現，解法二：迭代實現。

代碼實現

二叉樹結點類

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

 

算法實現類，解法一

import java.util.LinkedList;
import java.util.List;

public class Solution {

    private List<Integer> result;

    public List<Integer> inorderTraversal(TreeNode root) {
        result = new LinkedList<>();
        inOrder(root);
        return result;
    }

    private void inOrder(TreeNode root) {
        if (root != null) {
            inOrder(root.left);
            result.add(root.val);
            inOrder(root.right);
        }
    }
}

 

算法實現類，解法一

import java.util.LinkedList;
import java.util.List;

public class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {

        List<Integer> result = new LinkedList<>();
        LinkedList<TreeNode> stack = new LinkedList<>();
        TreeNode node = root;

        while (node != null || !stack.isEmpty()) {
            while (node != null) {
                stack.addLast(node);
                node = node.left;
            }

            if (!stack.isEmpty()) {
                node = stack.removeLast();
                result.add(node.val);
                node = node.right;
            }
        }

        System.out.println(result);
        return result;
    }
}