add solution 217[js]

Author: limbowandering

solution/217.Contains Duplicate/Solution.js

@@ -0,0 +1,23 @@
+const containsDuplicate = function(nums){
+  let set = new Set();
+  nums.forEach(e => {
+    set.add(e);
+  })
+  return set.size !== nums.length;
+}
+var containsDuplicate2 = function(nums) {
+  return (new Set(nums).size !== nums.length);    
+};
+var containsDuplicate3 = function(nums) {
+  const map = {}; // key => number, value => count
+
+  for (let i = 0; i < nums.length; i++) {
+    if (map[nums[i]] !== undefined) {
+      return true
+    } else {
+      map[nums[i]] = 1
+    }
+  }
+
+  return false
+};