题目

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给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

示例 1：

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输入：root = [1,null,2,3]
输出：[1,2,3]

示例 2：

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输入：root = []
输出：[]

解题思路

递归

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/**
 * 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 preorderTraversal = function(root) {
    let result = [];
    const dfs = function(root){
        if(root == null) return;
        result.push(root.val);
        dfs(root.left);
        dfs(root.right);
    }
    dfs(root);
    return result;
};

迭代

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/**
 * 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 preorderTraversal = function(root) {
    let result = [];
    if(!root) return result;
    // 根节点先入栈
    let stack = [root];
    let cur = null
    while(stack.length){
        // 栈顶出战
        cur = stack.pop();
        result.push(cur.val);
        // 中左右进栈
        cur.right && stack.push(cur.right);
        cur.left && stack.push(cur.left);
    }
    return result;
};