fd

Author: dingjikerbo

extras.md

@@ -1,4 +1,5 @@
 # 其它题目
 
 ### Google
- - [Remove consecutive node from single list](leetcode/solution/src/RemoveDuplicateList.java)
+ - [Remove consecutive node from single list](leetcode/solution/src/RemoveDuplicateList.java)
+ - [Reverse pairs](leetcode/solution/src/CountReversePairs.java)

leetcode/solution/src/CountReversePairs.java

@@ -0,0 +1,17 @@
+public class CountReversePairs {
+
+    /**
+     * 数组arr，[0,m)和[m,arr.length)两个区间都是递增的
+     * 如果[0,m)区间的数比[m, arr.len)区间大，则能构成一个reverse pair
+     * 问有多少个reverse pair
+     */
+    int reversePairs(int[] arr, int m) {
+        int pairs = 0;
+        for (int i = m - 1, j = arr.length - 1; j >= m; j--) {
+            for ( ; i >= 0 && arr[i] > arr[j]; i--);
+            pairs += m - 1 - i;
+        }
+        return pairs;
+    }
+}
+

leetcode/solution/src/RemoveDuplicateList.java

@@ -7,29 +7,51 @@ public class RemoveDuplicateList {
     public ListNode removeDup(ListNode head) {
         ListNode dummy = new ListNode(0), cur = dummy;
 
-        ListNode node = null;
+        ListNode last = null;
         int count = 0;
 
         for (ListNode p = head; p != null; p = p.next) {
-            if (node == null || node.val == p.val) {
-                node = p;
+            if (last == null || last.val == p.val) {
+                last = p;
                 count++;
             } else {
                 if (count == 1) {
-                    cur.next = node;
+                    cur.next = last;
                     cur = cur.next;
                 }
-                node = p;
+                last = p;
                 count = 1;
             }
         }
 
-        if (node != null && count == 1) {
-            cur.next = node;
+        if (last != null && count == 1) {
+            cur.next = last;
             cur = cur.next;
         }
 
         cur.next = null;
         return dummy.next;
     }
+
+    // 可在上面的基础上精简
+    // count是可以去掉的
+    public ListNode removeDup2(ListNode head) {
+        if (head == null) {
+            return null;
+        }
+        ListNode dummy = new ListNode(0), cur = dummy;
+        ListNode last = head;
+
+        for (ListNode p = head.next; p != null; p = p.next) {
+            if (last.val != p.val) {
+                if (last.next == p) {
+                    cur.next = last;
+                    cur = cur.next;
+                }
+                last = p;
+            }
+        }
+        cur.next = (last.next == null ? last : null);
+        return dummy.next;
+    }
 }

leetcode/src/Main.java

@@ -3,56 +3,23 @@ public class Main {
 
     public static class Solution {
 
-        ListNode removeDup(ListNode head) {
-            ListNode dummy = new ListNode(0), cur = dummy;
-
-            ListNode node = null;
-            int count = 0;
-
-            for (ListNode p = head; p != null; p = p.next) {
-                if (node == null || node.val == p.val) {
-                    node = p;
-                    count++;
-                } else {
-                    if (count == 1) {
-                        cur.next = node;
-                        cur = cur.next;
-                    }
-                    node = p;
-                    count = 1;
-                }
-            }
-
-            if (node != null && count == 1) {
-                cur.next = node;
-                cur = cur.next;
-            }
-
-            cur.next = null;
-            return dummy.next;
-        }
-
-        ListNode make(int[] arr) {
-            ListNode dummy = new ListNode(0), cur = dummy;
-            for (int n : arr) {
-                cur.next = new ListNode(n);
-                cur = cur.next;
+        int reversePairs(int[] arr, int m) {
+            int pairs = 0;
+            for (int i = m - 1, j = arr.length - 1; j >= m; j--) {
+                for ( ; i >= 0 && arr[i] > arr[j]; i--);
+                pairs += m - 1 - i;
             }
-            return dummy.next;
+            return pairs;
         }
-
     }
 
 
     public static void main(String[] args) {
         Solution s = new Solution();
         int[] arr = {
-                1, 3, 3, 3, 1, 2, 4, 4, 1
+                1, 4, 8, 9, 2, 3, 6, 7
         };
-        ListNode head = s.make(arr);
-        head = s.removeDup(head);
-        for (ListNode p = head; p != null; p = p.next) {
-            System.out.print(p.val + " ");
-        }
+        int n = s.reversePairs(arr, 4);
+        System.out.println(n);
     }
 }