feat(tests): add euler-206 solution (#46256)

curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-206-concealed-square.md

@@ -34,5 +34,30 @@ concealedSquare();
 # --solutions--
 
 ```js
-// solution required
+// Check if n**2 matches the pattern
+function squareMatchs(n) {
+  // Need BigInt due to size of values
+  let nSquared = (BigInt(n) * BigInt(n)).toString();
+
+  // Check if digits match pattern
+  for (let i = 1; i <= 9; i++) {
+    if (nSquared[2 * (i - 1)] != i) return false;
+  }
+  return true;
+}
+
+// Find integer whose square matches the pattern
+function concealedSquare() {
+  // Set bounds based upon max and min candidates
+  const minSquareRoot = Math.floor(Math.sqrt(10203040506070809) / 10) * 10;
+  const maxSquareRoot = Math.ceil(Math.sqrt(19293949596979899) / 10) * 10;
+
+  for (let x = maxSquareRoot; x >= minSquareRoot; x -= 10) {
+    // Note: 3*3 = 9 and 7*7 = 49 are only trailing digits
+    //       that can produce 9 as trailing digit in square
+    if (squareMatchs(x + 3)) return (x + 3)*10;
+    if (squareMatchs(x + 7)) return (x + 7)*10;
+  }
+  return -1;
+}
 ```