Add guessNumber function using binary search

Implement binary search to guess the number.

Author: jyoung2419

leetcode3/정진영/374. Guess Number Higher or Lower

@@ -0,0 +1,30 @@
+/** 
+ * Forward declaration of guess API.
+ * @param {number} num   your guess
+ * @return 	     -1 if num is higher than the picked number
+ *			      1 if num is lower than the picked number
+ *               otherwise return 0
+ * var guess = function(num) {}
+ */
+
+/**
+ * @param {number} n
+ * @return {number}
+ */
+var guessNumber = function(n) {
+    // 이진탐색
+    // 시간복잡도: O(log n)
+    let start = 1;
+    let end = n;
+    while (start <= end) {
+        let mid = Math.floor((start + end)/2);
+        if (guess(mid) == 1){ 
+            start = mid+1; 
+        } else if (guess(mid) == -1){ 
+            end = mid-1; 
+        } else {
+            return mid;
+            break;
+        }
+    }
+};