0094. 二叉树的中序遍历

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function (root) {
  const ret = [];

  const inorder = (node) => {
    if (!node) return;
    inorder(node.left);
    ret.push(node.val);
    inorder(node.right);
  };

  dfs(root);

  return ret;
};

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function (root) {
  const ret = [];
  const stack = [];

  while (root || stack.length > 0) {
    while (root) {
      stack.push(root);
      root = root.left;
    }
    root = stack.pop();
    ret.push(root.val);
    root = root.right;
  }

  return ret;
};