找到链表中环的入口节点

src/Question23.java

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+public class Question23 {
+    public static void main(String[] args) {
+        ListNode head = new ListNode(1);
+        ListNode node2 = new ListNode(2);
+        ListNode node3 = new ListNode(3);
+        ListNode node4 = new ListNode(4);
+        ListNode node5 = new ListNode(5);
+        head.next = node2;
+        node2.next = node3;
+        node3.next = node4;
+        node4.next = node5;
+        node5.next = node3;
+        System.out.println(enterNode(head).val);
+    }
+
+    static ListNode enterNode(ListNode head){
+        if (isContainLoop(head) != null){
+            return getEnterNode(head, getLoopLength(head, isContainLoop(head)));
+        }
+        return null;
+    }
+    //判断链表中是否有环,并返回环中的一个节点
+    static ListNode isContainLoop(ListNode head){
+        if (head == null || head.next == null){
+            return null;
+        }
+        ListNode slowOne = head;
+        ListNode fastOne = slowOne.next;
+
+        while (slowOne.next != null && fastOne.next != null){
+            //快指针每次走2步，慢指针每次走一步,当两个节点相遇则链表中含有环，且相遇节点为环中节点
+            if (slowOne.val == fastOne.val){
+                return fastOne;
+            }
+            fastOne = fastOne.next;
+            if (fastOne.val == slowOne.val){
+                return fastOne;
+            }
+            slowOne = slowOne.next;
+            fastOne = fastOne.next;
+        }
+
+        return null;
+    }
+    //得到环的长度
+    static int getLoopLength(ListNode head, ListNode meetNode){
+        boolean firstMeet = false;
+        int count = 0;
+        //从相遇节点开始计数，若再次相遇可得出环中节点数
+        while (head.next != null){
+            if (head.val == meetNode.val){
+                if (!firstMeet){
+                    count ++;
+                    firstMeet = true;
+                }else {
+                    break;
+                }
+            }else {
+                if (firstMeet){
+                    count ++;
+                }
+            }
+            head = head.next;
+        }
+        return count;
+    }
+    //得到环的入口节点
+    static ListNode getEnterNode(ListNode head, int loopLength){
+        /**1->2->3->4->5
+         *       |     |
+         *       -------
+         * **/
+        //快指针先走环长度的步数，此时快指针到入口的距离=起点到入口的距离
+        //假设起点到最远点长度为m,环长度为l,即m-l=m-(l+1)+1
+        ListNode fastNode = head;
+        ListNode slowNode = head;
+        while (loopLength > 0){
+            fastNode = fastNode.next;
+            loopLength --;
+        }
+        while (fastNode.val != slowNode.val){
+            fastNode = fastNode.next;
+            slowNode = slowNode.next;
+        }
+        return fastNode;
+    }
+    static class ListNode {
+        ListNode next;
+        int val;
+
+        public ListNode(int val) {
+            this.val = val;
+        }
+    }
+}