LeetCode 数、二叉树、二叉搜索树篇()

94. 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]
进阶: 递归算法很简单，你可以通过迭代算法完成吗？

//中序遍历 必会
//两个思路 递归、栈

solution1 递归

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List <Integer> res = new ArrayList <> ();
        helper(root, res);
        return res;
    }
    private void helper(TreeNode root, List <Integer> res) {
        if (root != null) {
            if (root.left != null) {
                helper(root.left, res);
            }
            res.add(root.val);
            if (root.right != null) {
                helper(root.right, res);
            }
        }
    }
}

solution2 栈

class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List <Integer> res = new ArrayList <> ();
        Stack <TreeNode> stack = new Stack<>();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()){
            //先把根结点放入stack
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            res.add(curr.val);
            curr = curr.right;
        }
        return res;
    }
}

LeetCode 数、二叉树、二叉搜索树篇()

原文：https://www.cnblogs.com/gg12138/p/13110848.html