94. Binary Tree Inorder Traversal 二叉树中序遍历

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

For example:
Given binary tree [1,null,2,3],

   1
    
     2
    /
   3

return [1,3,2].

Note: Recursive solution is trivial, could you do it iteratively?

题意：不使用递归的方式，中序遍历二叉树

/**
 * Definition for a binary tree node.
 * function TreeNode(val) {
 *     this.val = val;
 *     this.left = this.right = null;
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
let inorderTraversal = (root) => {
    let res = [];
    if (root == null) return res;
    let stack = [];
    var node = root;
    while (node != null || stack.length != 0) {
        while (node != null) {
            stack.push(node);
            node = node.left;
        }
        if (stack.length != 0) {
            node = stack.pop();
            res.push(node.val)
            node = node.right;
        }
    }
    return res;
}

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