751_IP_to_CIDR

Author: qiyuangong

README.md

@@ -178,6 +178,7 @@ Also, there are open source implementations for basic data structs and algorithm
 | 724 | [Find Pivot Index](https://leetcode.com/problems/find-pivot-index/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/724_Find_Pivot_Index.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/724_Find_Pivot_Index.java) | Seach the array to find a place where left sum is equal to right sum, O(n) and O(1) |
 | 733 | [Flood Fill](https://leetcode.com/problems/flood-fill/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/733_Flood_Fill.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/733_Flood_Fill.java) | 1. DFS with stack or recursive, O(n) and O(n)<br>2. BFS with queue, O(n) and O(n) |
 | 743 | [Network Delay Time](https://leetcode.com/problems/network-delay-time/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/743_Network_Delay_Time.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/743_Network_Delay_Time.java) | Let V == N, then: 1. DFS, O(V^V+ElgE), O(V+E)<br>2. Dijkstra, O(V^2+E), O(V+E)|
+| 751 | [IP to CIDR](https://leetcode.com/problems/ip-to-cidr/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/751_IP_to_CIDR.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/751_IP_to_CIDR.java) | Bit manipulations, incrementail is 1 << (32 - mask) |
 | 766 | [Toeplitz Matrix](https://leetcode.com/problems/toeplitz-matrix/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/766_Toeplitz_Matrix.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/766_Toeplitz_Matrix.java) | Check from top left to bottom right, i,j == i + 1, j + 1. |
 | 771 | [Jewels and Stones](https://leetcode.com/problems/jewels-and-stones/description/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/771_Jewels_and_Stones.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/771_Jewels_and_Stones.java) | Count given char in string. Hash or table. [Oneline](https://leetcode.com/problems/jewels-and-stones/discuss/113574/1-liners-PythonJavaRuby) |
 | 804 | [Unique Morse Code Words](https://leetcode.com/problems/unique-morse-code-words/description/) | [Python](https://github.com/qiyuangong/leetcode/blob/master/python/804_Unique_Morse_Code_Words.py) [Java](https://github.com/qiyuangong/leetcode/blob/master/java/804_Unique_Morse_Code_Words.java) | String, Hash and Set. Set is recommended. |

java/751_IP_to_CIDR.java

@@ -0,0 +1,34 @@
+class Solution {
+    public List<String> ipToCIDR(String ip, int n) {
+        long start = ipToLong(ip);
+        List<String> ans = new ArrayList();
+        while (n > 0) {
+            int mask = Math.max(33 - bitLength(Long.lowestOneBit(start)),
+                                33 - bitLength(n));
+            ans.add(longToIP(start) + "/" + mask);
+            start += 1 << (32 - mask);
+            n -= 1 << (32 - mask);
+        }
+        return ans;
+    }
+    private long ipToLong(String ip) {
+        long ans = 0;
+        for (String x: ip.split("\\.")) {
+            ans = 256 * ans + Integer.valueOf(x);
+        }
+        return ans;
+    }
+    private String longToIP(long x) {
+        return String.format("%s.%s.%s.%s",
+            x >> 24, (x >> 16) % 256, (x >> 8) % 256, x % 256);
+    }
+    private int bitLength(long x) {
+        if (x == 0) return 1;
+        int ans = 0;
+        while (x > 0) {
+            x >>= 1;
+            ans++;
+        }
+        return ans;
+    }
+}

python/751_IP_to_CIDR.py

@@ -0,0 +1,23 @@
+class Solution(object):
+    def ipToInt(self, ip):
+        ans = 0
+        for x in ip.split('.'):
+            ans = 256 * ans + int(x)
+        return ans
+
+    def intToIP(self, x):
+        return ".".join(str((x >> i) % 256)
+                        for i in (24, 16, 8, 0))
+
+    def ipToCIDR(self, ip, n):
+        # Start value of IP
+        start = self.ipToInt(ip)
+        ans = []
+        while n:
+            # Last 1 of start or can start from 0
+            mask = max(33 - (start & -start).bit_length(),
+                       33 - n.bit_length())
+            ans.append(self.intToIP(start) + '/' + str(mask))
+            start += 1 << (32 - mask)
+            n -= 1 << (32 - mask)
+        return ans