面试题02.07.链表相交：增加C语言实现

problems/面试题02.07.链表相交.md

@@ -249,6 +249,45 @@ var getIntersectionNode = function(headA, headB) {
 };
 ```
 
+C：
+
+```c
+ListNode *getIntersectionNode(ListNode *headA, ListNode *headB) {
+    ListNode *l = NULL, *s = NULL;
+    int lenA = 0, lenB = 0, gap = 0;
+    // 求出两个链表的长度
+    s = headA;
+    while (s) {
+        lenA ++;
+        s = s->next;
+    }
+    s = headB;
+    while (s) {
+        lenB ++;
+        s = s->next;
+    }
+
+    // 求出两个链表长度差
+    if (lenA > lenB) {
+        l = headA, s = headB;
+        gap = lenA - lenB;
+    } else {
+        l = headB, s = headA;
+        gap = lenB - lenA;
+    }
+
+    // 尾部对齐
+    while (gap--) l = l->next;
+    // 移动，并检查是否有相同的元素
+    while (l) {
+        if (l == s) return l;
+        l = l->next, s = s->next;
+    }
+
+    return NULL;
+}
+```
+