Add files via upload

Author: assassint2017

Combination Sum IV/Combination_Sum_IV.cpp

@@ -0,0 +1,57 @@
+// 典型的回溯法
+// 下面的这种暴力写法直接超时了
+
+class Solution 
+{
+public:
+    int combinationSum4(vector<int>& nums, int target) 
+    {
+        int res = 0;
+        int curSum = 0;
+
+        genSequence(nums, target, curSum, res);
+        return res;
+    }
+private:
+    void genSequence(const vector<int>& nums, const int& target, int& curSum, int& res)
+    {
+        if (curSum >= target)
+        {
+            if (curSum == target) 
+                ++res;
+            return ;
+        }
+
+        for (int i = 0; i < nums.size(); ++i)
+        {
+            curSum += nums[i];
+            genSequence(nums, target, curSum, res);
+            curSum -= nums[i];
+        }
+    }
+};
+
+// 转换为自底向上的动态规划来处理
+// Runtime: 4 ms, faster than 67.65% of C++ online submissions for Combination Sum IV.
+// Memory Usage: 8.6 MB, less than 96.67% of C++ online submissions for Combination Sum IV.
+class Solution 
+{
+public:
+    int combinationSum4(vector<int>& nums, int target) 
+    {
+        // 这里如果使用int就会溢出，long同样会溢出，换成无符号就没问题，这好奇怪呀
+        // 这里应该是确实结果太大的情况，答案就自动求余数了
+        vector<unsigned int> memo(target + 1, 0);
+        memo[0] = 1;
+        for (int i = 1; i <= target; ++i)
+        {
+            for (auto item : nums)
+            {
+                if (i - item >= 0)
+                    memo[i] += memo[i - item];
+            }
+        }
+
+        return memo[target];
+    }
+};

Letter Tile Possibilities/Letter_Tile_Possibilities.cpp

@@ -0,0 +1,39 @@
+// 第一种思路是使用哈希表，每产生一个序列就插入到哈希表中
+// Runtime: 60 ms, faster than 35.43% of C++ online submissions for Letter Tile Possibilities.
+// Memory Usage: 17.3 MB, less than 100.00% of C++ online submissions for Letter Tile Possibilities.
+
+typedef unordered_set<string> hashSet;
+
+class Solution 
+{
+public:
+    int numTilePossibilities(string tiles) 
+    {
+        unordered_set<string> hashset;
+        vector<bool> visited(tiles.length(), false);
+        string str = "";
+        
+        genSequence(tiles, visited, hashset, str);
+        return hashset.size();
+    }
+private:
+    void genSequence(const string& input, vector<bool>& visited, hashSet& hashset, string& str)
+    {
+        for (int i = 0; i < input.length(); ++i)
+        {
+            if (visited[i] == false)
+            {
+                str += input[i];
+                visited[i] = true;
+                
+                if (hashset.find(str) == hashset.end())
+                    hashset.insert(str);
+                
+                genSequence(input, visited, hashset, str);
+                
+                str.pop_back();
+                visited[i] = false;
+            }
+        }
+    }
+};