Hash:2

Author: rchen102

Java/leetcode 136.java

@@ -11,16 +11,19 @@ public int singleNumber(int[] nums) {
 }
 
 
-//Solution2: using HashMap T: O(n) S: O(n)
+// Solution2: HashMap 
+// T: O(n) S: O(n)
 class Solution {
     public int singleNumber(int[] nums) {
+        if (nums == null || nums.length == 0) return -1;
         Map<Integer, Integer> map = new HashMap<>();
-        for(int num : nums) {
-            if(map.containsKey(num)) map.put(num, 0);
+        for (int num : nums) {
+            if (map.containsKey(num)) map.put(num, 0);
             else map.put(num, 1);
         }
-        for(int num : nums)
-            if(map.get(num) == 1) return num;
-        return nums[0];
+        for (int num : nums) {
+            if (map.get(num) == 1) return num;
+        }
+        return -1;
     }
 }

Java/leetcode 187.java

@@ -1,4 +1,5 @@
-//Solution1: HashSet T: O(n) S: O(n)
+// Solution1: HashSet 
+// T: O(n) S: O(n)
 class Solution {
     public List<String> findRepeatedDnaSequences(String s) {
         Set<String> set = new HashSet<>();

Java/leetcode 202.java

@@ -1,16 +1,14 @@
 class Solution {
     public boolean isHappy(int n) {
-        Set<Integer> set = new HashSet<Integer>();
-        int sum = 0, tmp = 0;
-        
-        while(set.add(n)) {
-            sum = 0;
-            while(n > 0) {
-                tmp = n % 10;
+        Set<Integer> set = new HashSet<>();
+        while (set.add(n)) {
+            int sum = 0;
+            while (n != 0) {
+                int tmp = n % 10;
                 sum += tmp * tmp;
-                n /= 10;
+                n = n / 10;
             }
-            if(sum == 1) return true;
+            if (sum == 1) return true;
             n = sum;
         }
         return false;