114. 二叉树展开为链表.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 {void} Do not return anything, modify root in-place instead.
 */
var flatten = function (root) {
    const list = [];
    preorderTraversal(root, list);
    const size = list.length;
    for (let i = 1; i < size; i++) {
        const prev = list[i - 1],
            curr = list[i];
        prev.left = null;
        prev.right = curr;
    }
};

const preorderTraversal = (root, list) => {
    if (root != null) {
        list.push(root);
        preorderTraversal(root.left, list);
        preorderTraversal(root.right, list);
    }
}