Intersection of Two Linked Lists Problem & Solution

Given the heads of two singly linked-lists headA and headB, return the node at which the two lists intersect. If the two linked lists have no intersection at all, return null.

See the intersection of two linked lists problem on LeetCode.

C++ Solution

The runtime complexity of the algorithm is O ( n + m ) O(n + m) .

#pragma GCC optimize("Ofast")
#pragma GCC optimization("unroll-loops")

static const int _=[](){std::ios::sync_with_stdio(false);cin.tie(nullptr);cout.tie(nullptr);return 0;}();

/**
 * struct ListNode {
 *   int val;
 *   ListNode *next;
 *   ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
  ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
    int lenA = 0;
    ListNode* cursorA = headA;
    while (cursorA != nullptr) {
      cursorA = cursorA->next;
      ++lenA;
    }

    int lenB = 0;
    ListNode* cursorB = headB;
    while (cursorB != nullptr) {
      cursorB = cursorB->next;
      ++lenB;
    }

    while (lenA > lenB) {
      headA = headA->next;
      --lenA;
    }

    while (lenB > lenA) {
      headB = headB->next;
      --lenB;
    }

    // At this step, `lenA == lenB`.
    while (lenA > 0) {
      if (headA == headB) {
        return headA;
      }

      headA = headA->next;
      headB = headB->next;
      --lenA;
    }

    return nullptr;
  }
};

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