相交链表

输入两个链表，找出它们的第一个公共节点。

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

/**
 * @param {ListNode} headA
 * @param {ListNode} headB
 * @return {ListNode}
 */
var getIntersectionNode = function(headA, headB) {
    if (!headA || !headB) return null;
    let pA = headA;
    let pB = headB;
    while(pA != pB) {
        pA = pA == null ? headB : pA.next;
        pB = pB == null ? headA : pB.next;
    }
    return pA;
};

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