94.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) {
    let res = []
    let stack = []
    while (root || stack.length) {
        while (root) {
            stack.push(root)
            root = root.left
        }
        root = stack.pop()
        res.push(root.val)
        root = root.right
    }
    return res
}

/* var inorderTraversal = function (root) {
    let ans = []
    inOrder(root, ans)
    return ans
}

let inOrder = (root, res) => {
    inOrder(root.left, ans)
    ans.push(root.val)
    inOrder(root.right, ans)
} */

/*
    2021/8/8
    57% 99%
    好像递归是省空间些
    2021/10/3
    补充迭代解法
*/