Update leetcode 875 example with comments and with differnt approachs

Author: PhysicsX

Cpp/kokoEatingBananas.cpp

@@ -1,9 +1,40 @@
 #include <iostream>
 #include <vector>
 #include <algorithm>
+#include <cmath>
+
+// leetcode 875. Koko Eating Bananas
+
+// clasic binary search example
+
+// Piles = [3, 6, 7, 11] How many number of bananas should be ate per hour to finish in H hour all piles.
+// we can eat 1 banana per hour then total hours will be 27 . 
+// we can eat 2 bananas per hour then total hours will be 15 hours.
+// maximum number of bananas per hour can be the std::max_element(piles.begin(), piles.end())
+// in this case, 11. If 11 bananas is ate per one hour then all piles will finish in 4 hours.
+/*
+with 1 bananas per hour, all piles will finish in 27 hours
+with 2 bananas per hour, all piles will finish in 15 hours
+with 3 bananas per hour, all piles will finish in 10 hours
+with 4 bananas per hour, all piles will finish in 8 hours
+with 5 bananas per hour, all piles will finish in 8 hours
+with 6 bananas per hour, all piles will finish in 6 hours
+with 7 bananas per hour, all piles will finish in 5 hours
+with 8 bananas per hour, all piles will finish in 5 hours
+with 9 bananas per hour, all piles will finish in 5 hours
+with 10 bananas per hour, all piles will finish in 5 hours
+with 11 bananas per hour, all piles will finish in 4 hours
+
+As it is seen, if number of bananas to be ate for per hour is increasing then the
+total time to finish all piles is decreasing. This is called monotonic function.
+With monotonic function we can use binary search to find desired value.
+(brute force will take much time)
+
+simply we have array 1 to 11, it is needed to choose correct number according to the desired hour.
+*/
 
 class Solution {
-public:
+
     long long findSuitableHour(const std::vector<int>& piles, int target)
     {
          long long currPile = 0;
@@ -24,7 +55,19 @@ class Solution {
          }
          return hours;        
     }
-    
+
+    long long hoursToFinish(std::vector<int>& piles, int bananasForOneHour)
+    {
+        long long totalHour {0};
+
+        for(const auto& pile : piles)
+        {
+            totalHour += ceil(pile/(double)bananasForOneHour);
+        }
+        return totalHour;
+    }
+
+public:
     int minEatingSpeed(std::vector<int>& piles, int h) {
 
         int l=1, r= *max_element(piles.begin(),piles.end());;
@@ -39,7 +82,32 @@ class Solution {
         }
 
         return l;
-    }   
+    }
+
+    int minEatingSpeed_2(std::vector<int>& piles, int hour)
+    {
+        int max {*std::max_element(piles.begin(), piles.end())};
+        int low {1};
+
+        int ans {0};
+
+        while(low <= max)
+        {
+            int mid = low + (max - low)/2;
+            if(hoursToFinish(piles, mid) <= hour)
+            {
+                ans = mid;
+                max = mid - 1;
+            }
+            else
+            {
+                low = mid + 1;
+            }
+        }
+
+        return ans;
+    }
+
     int minEatingSpeedBruteForce(const std::vector<int>& piles, int target) {
 
      for(unsigned int j = 1; j <= *max_element(piles.begin(),piles.end()); j++)
@@ -77,6 +145,7 @@ int main()
 {
 	std::vector<int> vec {30,11,23,4,20};
 	std::cout<<Solution{}.minEatingSpeed(vec, 6)<<std::endl;
+	std::cout<<Solution{}.minEatingSpeed_2(vec, 6)<<std::endl;
 
 	std::cout<<Solution{}.minEatingSpeedBruteForce(vec, 6)<<std::endl;