非常好理解的解法

先遍历链表1，将flag设为1；再遍历链表2，将flag设为0，在找出一个flag为0的节点就是公共节点

function ListNode(x){
    this.val = x;
    this.next = null;
}
function FindFirstCommonNode(pHead1, pHead2)
{
    // write code here
    let head1 = new ListNode(0);
    head1.next = pHead1;
    while (pHead1) {
        pHead1.flag = 1;
        pHead1 = pHead1.next;
    }
    while (pHead2) {
        pHead2.flag = 0;
        pHead2 = pHead2.next;
    }
    while (head1.next) {
        if (head1.next.flag == 0) {
            return head1.next
        }
        head1 = head1.next;
    }
    return null
}
module.exports = {
    FindFirstCommonNode : FindFirstCommonNode
};