94. Binary Tree Inorder Traversal（二叉树的中序遍历）

题目描述

方法思路

Approach1: recursive

class Solution {
    //Runtime: 0 ms, faster than 100.00%
    //Memory Usage: 36.3 MB, less than 11.58%
    List<Integer> res = new LinkedList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null) return res;
        inorderTraversal(root.left);
        res.add(root.val);
        inorderTraversal(root.right);
        return res;
    }
}

Approach2: iteratively

public class Solution {
    //Runtime: 0 ms, faster than 100.00%
    //Memory Usage: 36.1 MB, less than 66.62%
    public List < Integer > inorderTraversal(TreeNode root) {
        List < Integer > res = new ArrayList < > ();
        Stack < TreeNode > stack = new Stack < > ();
        TreeNode curr = root;
        while (curr != null || !stack.isEmpty()) {
            while (curr != null) {
                stack.push(curr);
                curr = curr.left;
            }
            curr = stack.pop();
            res.add(curr.val);
            curr = curr.right;
        }
        return res;
    }
}