Create Minimum Cost to Merge Stones.cpp

Dynamic Programming/Minimum Cost to Merge Stones.cpp

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+//recursion
+class Solution {
+public:
+    int recurr(int i,int j,int k,vector<int> &prefix,vector<int> &stones){
+        if(j-i<=k-2) return 0;
+        int mini = INT_MAX;
+        for(int t=i;t<j;t+=(k-1)){
+            int left = recurr(i,t,k,prefix,stones);
+            int right = recurr(t+1,j,k,prefix,stones);
+            mini = min(mini,(left+right));
+        }
+        if((j-i)%(k-1)==0){
+            int sum=0;
+            sum+=prefix[j];
+            if(i>0){
+                sum-=prefix[i-1];
+            }
+            mini+=sum;
+        }
+        return mini;
+    }
+
+    int mergeStones(vector<int>& stones, int k) {
+        int n = stones.size();
+        if((n-1)%(k-1)) return -1;
+        vector<int> prefix(n,0);
+        prefix[0] = stones[0];
+        for(int i=1;i<n;i++){
+            prefix[i] = prefix[i-1]+stones[i];
+        }
+        return recurr(0,n-1,k,prefix,stones);
+    }
+};
+
+//memoization
+class Solution {
+public:
+    int recurr(int i,int j,int k,vector<int> &prefix,vector<int> &stones,vector<vector<int>> &dp){
+        if(j-i<=k-2) return 0;
+        int mini = INT_MAX;
+        if(dp[i][j]!=-1) return dp[i][j];
+        for(int t=i;t<j;t+=(k-1)){
+            int left = recurr(i,t,k,prefix,stones,dp);
+            int right = recurr(t+1,j,k,prefix,stones,dp);
+            mini = min(mini,(left+right));
+        }
+        if((j-i)%(k-1)==0){
+            int sum=0;
+            sum+=prefix[j];
+            if(i>0){
+                sum-=prefix[i-1];
+            }
+            mini+=sum;
+        }
+        return dp[i][j]=mini;
+    }
+
+    int mergeStones(vector<int>& stones, int k) {
+        int n = stones.size();
+        if((n-1)%(k-1)) return -1;
+        vector<int> prefix(n,0);
+        prefix[0] = stones[0];
+        for(int i=1;i<n;i++){
+            prefix[i] = prefix[i-1]+stones[i];
+        }
+        vector<vector<int>> dp(n,vector<int>(n,-1));
+        return recurr(0,n-1,k,prefix,stones,dp);
+    }
+};