递归版本

    1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number[]}
    12.  */
    13. var inorderTraversal = function(root) {
    14.     const res = [];
    15.     const inOrder = (root) => {
    16.         if(!root) return [];
    17.         inOrder(root.left);
    18.         res.push(root.val);
    19.         inOrder(root.right);
    20.     }
    21.     inOrder(root)
    22.     return res
    23. };

    非递归版本

    1. /**
    2.  * Definition for a binary tree node.
    3.  * function TreeNode(val, left, right) {
    4.  *     this.val = (val===undefined ? 0 : val)
    5.  *     this.left = (left===undefined ? null : left)
    6.  *     this.right = (right===undefined ? null : right)
    7.  * }
    8.  */
    9. /**
    10.  * @param {TreeNode} root
    11.  * @return {number[]}
    12.  */
    13. var inorderTraversal = function(root) {
    14.     if(!root) return [];
    15.     const stack = [];
    16.     const res = [];
    17.     let p = root;
    18.     while(stack.length || p) {
    19.         while(p) {
    20.             stack.push(p);
    21.             p = p.left;
    22.         }
    23.         const n = stack.pop();
    24.         res.push(n.val);
    25.         p = n.right;
    26.     }
    27.     return res
    28. };