Leetcode144

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144. 二叉树的前序遍历

总体思路递归，easy等级

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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) {
    const ans = [];
    recursion(root, ans);
    return ans
};
function recursion(root, ans) {
    if (root === null) return;
    ans.push(root.val);
    recursion(root.left, ans)
    recursion(root.right, ans)
}