leetcode new problems

leetcode/lc18/notes.md

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+Should be a hard problem.
+Core Idea : Fix one number, reduce the problem size, repeat until only two numbers are left, then solve that directly
+
+v.end() : 
+A member function
+Exists only if v is a container class
+
+end(v) : 
+A free function (std::end) which internally calls v.end(), safer, modern and generic
+
+Main difference:
+```
+int arr[] = {1,2,3};
+
+auto a = end(arr);   //  works
+auto b = arr.end(); //  ERROR
+
+```
+