Add majorityElement function to find majority number

Author: jyoung2419

leetcode3/정진영/169. Majority Element

@@ -0,0 +1,22 @@
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var majorityElement = function(nums) {
+    let ans = nums[0];
+    let count = 0;
+
+    for (let i = 0; i < nums.length; i++) {
+        if (count === 0) {
+            ans = nums[i];
+        }
+
+        if (nums[i] === ans) {
+            count++;
+        } else {
+            count--;
+        }
+    }
+
+    return ans;
+};