Add 668_Kth_Smallest_Number_in_Multiplication_Table (qiyuangong#21)

* Add 668_Kth_Smallest_Number_in_Multiplication_Table CPP solution, by @ruchit2801

Author: ruchit2801

cpp/668_Kth_Smallest_Number_in_Multiplication_Table.cpp

@@ -0,0 +1,28 @@
+class Solution {
+public:
+    #define ll long long int
+    
+    bool valid(ll x, int m, int n, int k){
+        int cnt=0;
+        for(int i=1;i<=m;i++){
+            cnt+=n<x/i?n:x/i;
+            if(x/i==0)break;
+        }
+        return cnt>=k;
+    }
+    
+    int findKthNumber(int n1, int n2, int k) {
+        ll l=0, r=n1*n2,ans;
+        while(l<=r){
+            ll m = l +(r-l)/2;
+            if(valid(m,n1,n2,k)){
+                ans=m;
+                r=m-1;
+            }
+            else{
+                l=m+1;
+            }
+        }
+        return ans;
+    }
+};