added 65.Valid Number soln

Author: sarthakjalan05

65.Valid Number.cpp

@@ -0,0 +1,52 @@
+#include <bits/stdc++.h>
+using namespace std;
+
+class ValidNumber {
+public:
+    bool isNumber(string s) {
+        int n = s.size();
+        int i = 0;
+
+        // Remove leading whitespaces
+        while (i < n && s[i] == ' ') i++;
+
+        // Remove trailing whitespaces
+        int j = n - 1;
+        while (j >= 0 && s[j] == ' ') j--;
+
+        if (i > j) return false;
+
+        bool num = false;   // At least one digit before e
+        bool dot = false;   // Only one '.'
+        bool exp = false;   // Only one 'e' or 'E'
+
+        for (int k = i; k <= j; k++) {
+            char c = s[k];
+
+            if (isdigit(c)) {
+                num = true;
+            } 
+            else if (c == '.') {
+                // Only one '.' and it can't appear after e
+                if (dot || exp) return false;
+                dot = true;
+            } 
+            else if (c == 'e' || c == 'E') {
+                // e must appear only once and must follow a number
+                if (exp || !num) return false;
+                exp = true;
+                num = false; // Reset, we need digits after e
+            } 
+            else if (c == '+' || c == '-') {
+                // Sign must be at start or immediately after e
+                if (k != i && s[k-1] != 'e' && s[k-1] != 'E')
+                    return false;
+            } 
+            else {
+                return false;
+            }
+        }
+
+        return num;
+    }
+};