[Function add]

1. Add leetcode solutions.

Author: Seanforfun

README.md

@@ -347,6 +347,12 @@
 
 [200. Number of Islands](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/200.%20Number%20of%20Islands.md)
 
+[201. Bitwise AND of Numbers Range](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/201.%20Bitwise%20AND%20ofvNumbers%20Range.md)
+
+[202. Happy Number](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/202.%20Happy%20Number.md)
+
+[203. Remove Linked List Elements](https://github.com/Seanforfun/Algorithm-and-Leetcode/blob/master/leetcode/203.%20Remove%20Linked%20List%20Elements.md)
+
 ## Algorithm(4th_Edition)
 Reading notes of book Algorithm(4th Algorithm),ISBN: 9787115293800.
 All java realization codes are placed in different packages.

leetcode/201. Bitwise AND of Numbers Range.md

@@ -34,5 +34,20 @@ class Solution {
 }
 ```
 
+### 二刷
+```Java
+class Solution {
+    public int rangeBitwiseAnd(int m, int n) {
+        int mask = ~0;
+        while(m != n){
+            m &= mask;
+            n &= mask;
+            mask <<= 1;
+        }
+        return m;
+    }
+}
+```
+
 ### Reference
 1.[[LeetCode] Bitwise AND of Numbers Range 数字范围位相与](https://www.cnblogs.com/grandyang/p/4431646.html)

leetcode/202. Happy Number.md

@@ -44,4 +44,35 @@ class Solution {
         }
     }
 }
+```
+
+### 二刷
+1. 二刷使用了循环，还是要用一个set去存储已经访问的数字。
+```Java
+class Solution {
+    public boolean isHappy(int n) {
+        if(n <= 0) return false;
+        Set<Integer> visited = new HashSet<>();
+        List<Integer> temp = null;
+        while(!visited.contains(n)){
+            visited.add(n);
+            temp = new ArrayList<>();
+            getDigits(temp, n);
+            int count = 0;
+            for(Integer d : temp){
+                count += d * d;
+            }
+            if(count == 1) return true;
+            n = count;
+        }
+        return false;
+    }
+    
+    private void getDigits(List<Integer> list, int n){
+        while(n != 0){
+            list.add(n % 10);
+            n /= 10;
+        }
+    }
+}
 ```

leetcode/203. Remove Linked List Elements.md

@@ -0,0 +1,69 @@
+## 203. Remove Linked List Elements
+
+### Question
+Remove all elements from a linked list of integers that have value val.
+
+Example:
+```
+Input:  1->2->6->3->4->5->6, val = 6
+Output: 1->2->3->4->5
+```
+
+### 二刷:
+* Method 1: 创建一个新的结点，如果符合要求则向这个结点后添加。
+```Java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode removeElements(ListNode head, int val) {
+        ListNode dummy = new ListNode(0);
+        ListNode dummyCur = dummy;
+        ListNode cur = head;
+        while(cur != null){
+            if(cur.val != val){
+                dummyCur.next = cur;
+                dummyCur = dummyCur.next;
+            }
+            cur = cur.next;
+        }
+        dummyCur.next = null;
+        return dummy.next;
+    }
+}
+```
+
+* Method 2: 原地去除结点，所以要记录前结点。
+```Java
+/**
+ * Definition for singly-linked list.
+ * public class ListNode {
+ *     int val;
+ *     ListNode next;
+ *     ListNode(int x) { val = x; }
+ * }
+ */
+class Solution {
+    public ListNode removeElements(ListNode head, int val) {
+        ListNode dummy = new ListNode(0);
+        dummy.next = head;
+        ListNode pre = dummy;
+        ListNode cur = head;
+        while(cur != null){
+            while(cur != null && cur.val == val){
+                cur = cur.next;
+            }
+            pre.next = cur;
+            if(cur == null) return dummy.next;
+            pre = pre.next;
+            cur = cur.next;
+        }
+        return dummy.next;
+    }
+}
+```