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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode getIntersectionNode(ListNode headA, ListNode headB) {
        if (headA == null || headB == null) {
            return null;
        }
        ListNode indexA = headA;
        ListNode indexB = headB;
        
        while (indexA != indexB) {
            if (indexA == null) {
                indexA = headB;
            } else { // 为null这一步不往下走
                indexA = indexA.next;
            }
            
            if (indexB == null) {
                indexB = headA;
            } else { // 为null这一步不往下走
                indexB = indexB.next;
            }
            
        }
        return indexA;
    }
}