added solution guessNumber

Author: MaryNfs

Go/Leetcode/guess_number/guess_number.go

@@ -1,6 +1,35 @@
 package guess_number
 
-// TODO: implement solution
-func Solve() {
-	// implement
+/**
+ * Forward declaration of guess API.
+ * @param  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
+ * func guess(num int) int;
+ */
+
+func guess(num int, pick int) int {
+	if num > pick {
+		return -1
+	} else if num < pick {
+		return 1
+	}
+	return 0
+}
+
+func guessNumber(n int, p int) int {
+	l, r := 1, n
+	for true {
+		m := (l + r) / 2
+		res := guess(m, p)
+		if res == 0 {
+			return m
+		} else if res > 0 {
+			l = m + 1
+		} else if res < 0 {
+			r = m - 1
+		}
+	}
+	return 1
 }