20-06-02_inorderTraversal.js

/**
 * 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 res = [];
  const stk = [];
  while (root || stk.length) {
      while (root) {
          stk.push(root);
          root = root.left;
      }
      root = stk.pop();
      res.push(root.val);
      root = root.right;
  }
  return res;
};

// 递归的思路
var inorderTraversal = function(root) {
  const res = [];
  const inorder = (root) => {
      if (!root) {
          return;
      }
      inorder(root.left);
      res.push(root.val);
      inorder(root.right);
  }
  inorder(root);
  return res;
};