題目鏈接: https://leetcode-cn.com/problems/middle-of-the-linked-list/
思路：用快慢指針期升，快指針每次走2步，慢指針每次走1步烙样，路程相同（鏈表長度一定）的情況下，快指針的速度是慢指針的2倍蕊肥，當(dāng)快指針走完鏈表的時(shí)候谒获，慢指針在鏈表的中間點(diǎn)蛤肌。
var middleNode = function(head) {
slow = fast = head;
while (fast && fast.next) { // 注意判斷條件，fast是會(huì)先遍歷完鏈表的批狱，要保證fast.next不為空
slow = slow.next;
fast = fast.next.next;
}
return slow;
};
