BM23 二叉树的前序遍历 ​

function preOrder(root: TreeNode, list: number[]): void {
    if(!root) return;
    list.push(root.val);
    if(root.left) preOrder(root.left, list);
    if(root.right) preOrder(root.right, list)
}

export function preorderTraversal(root: TreeNode): number[] {
    // write code here
    if(!root) return [];
    var list: number[] = [];
    preOrder(root, list)
    return list;
}

/*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 preorderTraversal(root: TreeNode): number[] {
    // write code here
    var res = []
    var stack: TreeNode[] = [];
    if(!root) return res;
    stack.push(root)
    
    while(stack.length) {
        let node = stack.pop();
        res.push(node.val);
        if(node.left) stack.push(node.left); 
        if(node.right) stack.push(node.right);       
    }
    return res;
}