/*class TreeNode {
 *     val: number
 *     left: TreeNode | null
 *     right: TreeNode | null
 *     constructor(val?: number, left?: TreeNode | null, right?: TreeNode | null) {
 *         this.val = (val===undefined ? 0 : val)
 *         this.left = (left===undefined ? null : left)
 *         this.right = (right===undefined ? null : right)
 *     }
 * }
 */
/**
 * 代码中的类名、方法名、参数名已经指定,请勿修改,直接返回方法规定的值即可
 *
 * 
 * @param root TreeNode类 
 * @return int整型一维数组
 */
export function inorderTraversal(root: TreeNode): number[] {
    const resArr: number[] = []
    if (root == null) return resArr
     
    // 解法一:递归
//     inOrder(root, resArr)
     
    // 面试官:用非递归解决?
     
    // 解法二:非递归
    inOrder2(root, resArr)
     
    return resArr
}

// 解法一:递归函数
// 时间复杂度 O(n)
function inOrder(node: TreeNode, resArr: number[]) {
    if (node == null) return
    inOrder(node.left, resArr)
    resArr.push(node.val)
    inOrder(node.right, resArr)
}

// 解法二:非递归函数
// 时间复杂度 O(n)
function inOrder2(root: TreeNode, resArr: number[]) {
    let node: TreeNode = root
    const stack: TreeNode[] = []
    while (stack.length || node) {
        while (node) {
            stack.push(node)
            node = node.left
        }
        const top = stack.pop()
        resArr.push(top.val)
        node = top.right
    }
}