Binary Tree Inorder Traversal

https://leetcode.com/problems/binary-tree-inorder-traversal/description/

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

Thoughts

和preorder基本一样。把所有左节点压入栈中, 区别在于preorder是结果（加入res list的顺序）跟着cur走。而inorder是结果跟着弹出的顺序走，因为弹出意味着它的左子树都已遍历完，该root结点了。

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    private void pushLefts(Stack<TreeNode> stack, TreeNode node) {
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
    }

    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> res = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        pushLefts(stack, root);
        while (!stack.isEmpty()) {
            TreeNode node = stack.pop();
            res.add(node.val);
            pushLefts(stack, node.right);
        } 
        return res;
    }
}

Analysis

时空复杂度都是O(N).