672 solved.

Author: liuyubobobo

0672-Bulb-Switcher-II/cpp-0672/CMakeLists.txt

@@ -0,0 +1,7 @@
+cmake_minimum_required(VERSION 3.5)
+project(cpp_0672)
+
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+
+set(SOURCE_FILES main2.cpp)
+add_executable(cpp_0672 ${SOURCE_FILES})

0672-Bulb-Switcher-II/cpp-0672/main.cpp

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+/// Source : https://leetcode.com/problems/bulb-switcher-ii/description/
+/// Author : liuyubobobo
+/// Time   : 2017-12-02
+
+#include <iostream>
+#include <unordered_set>
+
+using namespace std;
+
+/// We can see the first six bulb status can decide all the sequence
+/// So we can try all 2^4 operations and see how many status are there
+/// for the first six bulb
+///
+/// Time Complexity: O(2^4)
+/// Space Complexity: O(1)
+class Solution {
+public:
+    int flipLights(int n, int m) {
+
+        unordered_set<int> states;
+        n = min(n, 6);
+        for(int i = 0; i < 16 ; i++){
+            int state = 0b111111;
+
+            int onebit = onenum(i, 4);
+            if(onebit > m || m % 2 != onebit % 2)
+                continue;
+
+            if(i&1)
+                state ^= 0b111111;
+            if(i&2)
+                state ^= 0b101010;
+            if(i&4)
+                state ^= 0b010101;
+            if(i&8)
+                state ^= 0b001001;
+
+            states.insert(state&((1<<n)-1));
+        }
+
+        return states.size();
+    }
+
+private:
+    int onenum(int x, int bitnum){
+        int res = 0;
+        for(int i = 0 ; i < bitnum ; i ++)
+            if(x&(1<<i))
+                res ++;
+        return res;
+    }
+};
+
+int main() {
+
+    cout << Solution().flipLights(1, 1) << endl;
+    cout << Solution().flipLights(2, 1) << endl;
+    cout << Solution().flipLights(3, 1) << endl;
+    cout << Solution().flipLights(3, 3) << endl;
+    cout << Solution().flipLights(3, 2) << endl;
+
+    return 0;
+}

0672-Bulb-Switcher-II/cpp-0672/main2.cpp

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+/// Source : https://leetcode.com/problems/bulb-switcher-ii/description/
+/// Author : liuyubobobo
+/// Time   : 2017-12-02
+
+#include <iostream>
+#include <unordered_set>
+
+using namespace std;
+
+/// For further analysis, we can see the first three lights uniquely determine the rest of the sequence
+/// Assume the four operation is a, b, c, d, then:
+/// - Light 1 = 1 + a + c + d
+/// - Light 2 = 1 + a + b
+/// - Light 3 = 1 + a + c
+/// - Light 4 = 1 + a + b + d
+/// - Light 5 = 1 + a + c
+/// - Light 6 = 1 + a + b
+///
+/// so that (module 2)
+/// - Light 4 = (Light 1) + (Light 2) + (Light 3)
+/// - Light 5 = Light 3
+/// - Light 6 = Light 2
+///
+/// so there's just 8 cases, we can enumerate all these 8 cases and get the result.
+///
+/// Time Complexity: O(1)
+/// Space Complexity: O(1)
+class Solution {
+public:
+    int flipLights(int n, int m) {
+
+        n = min(n, 3);
+        if (m == 0) return 1;
+        if (m == 1) return n == 1 ? 2 : n == 2 ? 3 : 4;
+        if (m == 2) return n == 1 ? 2 : n == 2 ? 4 : 7;
+        return n == 1 ? 2 : n == 2 ? 4 : 8;
+    }
+};
+
+int main() {
+
+    cout << Solution().flipLights(1, 1) << endl;
+    cout << Solution().flipLights(2, 1) << endl;
+    cout << Solution().flipLights(3, 1) << endl;
+    cout << Solution().flipLights(3, 3) << endl;
+    cout << Solution().flipLights(3, 2) << endl;
+
+    return 0;
+}

readme.md

@@ -175,6 +175,7 @@ email: [[email protected]](mailto:[email protected])
 | | | | | | |
 | 648 | [Replace Words](https://leetcode.com/problems/replace-words/description/) | [无] | [C++](0648-Replace-Words/cpp-0648/) | | |
 | | | | | | |
+| 672 | [Bulb Switcher II](https://leetcode.com/problems/bulb-switcher-ii/description/) | [solution](https://leetcode.com/problems/bulb-switcher-ii/solution/) | [C++](0672-Bulb-Switcher-II/cpp-0672/) | | |
 | 673 | [Number of Longest Increasing Subsequence](https://leetcode.com/problems/number-of-longest-increasing-subsequence/description/) | [缺：BIT；线段树] | [C++](0673-Number-of-Longest-Increasing-Subsequence/cpp-0673/) | | |
 | 674 | [Longest Continuous Increasing Subsequence](https://leetcode.com/problems/longest-continuous-increasing-subsequence/description/) | | [C++](0674-Longest-Continuous-Increasing-Subsequence/cpp-0674/) | | |
 | 675 | [Cut Off Trees for Golf Event](https://leetcode.com/problems/cut-off-trees-for-golf-event/description/) | [缺：A*；Hadlock's Algo] | [C++](0675-Cut-Off-Trees-for-Golf-Event/cpp-0675/) | | |