Hash: 3

Author: rchen102

Java/leetcode 274.java

@@ -1,19 +1,20 @@
-//Own solution: sort and loop T: O(nlogn) S: O(logn)--O(n)
+// Solution1: Sort 
+// T: O(nlogn) S: O(1) (no count sort)
 class Solution {
     public int hIndex(int[] citations) {
+        if (citations == null || citations.length == 0) return 0;
         Arrays.sort(citations);
-        int len = citations.length;
-        int res = 0;
-        for(int i = len - 1; i >= 0; i--) {
-            if(citations[i] >= len - i) res = len - i;
-            else break;
+        for (int i = 0; i < citations.length; i++) {
+            int possibleH = citations.length - i;
+            if (citations[i] >= possibleH) return possibleH;
         }
-        return res;
+        return 0;
     }
 }
 
 
-//Solution1: loop T: O(n) S: O(n)
+// Solution2: Array(Hash)
+// T: O(n) S: O(n)
 class Solution {
     public int hIndex(int[] citations) {
         int len = citations.length;

Java/leetcode 290.java

@@ -1,46 +1,22 @@
-//Own solution: HashMap & HashSet T: O(n) S: O(n)
+// Solution1: HashMap
+// T: O(n) S: O(n)
 class Solution {
     public boolean wordPattern(String pattern, String str) {
-        Map<Character, String> map = new HashMap<>();
-        Set<String> set = new HashSet<>();
-        List<String> list = getStrList(str);
-        if(pattern.length() != list.size()) return false;
-        for(int i = 0; i < pattern.length(); i++) {
-            char s = pattern.charAt(i);
-            if(map.containsKey(s)) {
-                if(!map.get(s).equals(list.get(i))) return false; 
-            } else {
-                if(!set.add(list.get(i))) return false;
-                map.put(s, list.get(i));
-            }
-        }
-        return true;
-    }
-    
-    public List<String> getStrList(String str) {
-        List<String> list = new ArrayList<>();
-        int start = 0;
-        for(int j = 0; j < str.length(); j++) {
-            if(str.charAt(j) == ' ') {
-                list.add(str.substring(start, j));
-                start = j + 1;
-            } 
-        }
-        list.add(str.substring(start, str.length())); // Do not forget to add final element
-        return list;
-    }
-}
-
-//Solution1: more concise
-class Solution {
-    public boolean wordPattern(String pattern, String str) {
-        String[] list = str.split(" ");
-        if(pattern.length() != list.length) return false;
-        Map<Character, Integer> map = new HashMap<>();
-        Map<String, Integer> map2 = new HashMap<>();
+        if (pattern == null || str == null) return false;
+        char[] chs = pattern.toCharArray();
+        String[] strs = str.split(" ");
+        if (strs.length != chs.length) return false;
+        
+        Map<Character, Integer> mapChar = new HashMap<>();
+        Map<String, Integer> mapStr = new HashMap<>();
 
-        for(Integer i = 0; i < pattern.length(); i++) {
-            if(map.put(pattern.charAt(i), i) != map2.put(list[i], i)) return false;
+        for (int i = 0; i < strs.length; i++) {
+            int idx1 = mapChar.getOrDefault(chs[i], -1);
+            int idx2 = mapStr.getOrDefault(strs[i], -1);
+            
+            if (idx1 != idx2) return false;
+            mapChar.put(chs[i],i);
+            mapStr.put(strs[i],i);
         }
         return true;
     }

Java/leetcode 299.java

@@ -1,50 +1,58 @@
-//Solution1: using array T: O(n) S: O(1)
+// Solution1: HashMap 
+// T: O(2n) S: O(10)
 class Solution {
     public String getHint(String secret, String guess) {
-        int length = secret.length();
+        if (secret == null || guess == null) return null;
+        int len = secret.length();
         int bulls = 0, cows = 0;
-        int[] numbers = new int[10];
+        int[] count = new int[10];
 
-        for(int i = 0; i < length; i++) {
+        for (int i = 0; i < len; i++) {
             char s = secret.charAt(i);
             char g = guess.charAt(i);
-
-            if(s == g) bulls++;
-            else {
-                if(numbers[s - '0']++ < 0) cows++;
-                if(numbers[g - '0']-- > 0) cows++;
+            if (s == g) bulls++;
+            else count[s-'0']++;
+        }
+        
+        for (int i = 0; i < len; i++) {
+            char s = secret.charAt(i);
+            char g = guess.charAt(i);
+            if (s != g && count[g-'0'] > 0) {
+                cows++;
+                count[g-'0']--;
             }
         }
-        return bulls  + "A" + cows + "B";
+        return bulls + "A" + cows + "B";
     }
 }
 
-
-//Own way: using HashMap T: O(n) S: O(n)
+// Solution2: HashMap (Better version) 
+// T: O(n) S: O(10)
 class Solution {
     public String getHint(String secret, String guess) {
-        int length = secret.length();
+        if (secret == null || guess == null) return null;
+        int len = secret.length();
         int bulls = 0, cows = 0;
-        Map<Character, Integer> map = new HashMap<>();
-        
-        for(int i = 0; i < length; i++) {
-            char tmp = secret.charAt(i);
-            map.put(tmp, map.getOrDefault(tmp, 0) + 1);
-            
-            if(tmp == guess.charAt(i)) {
-                bulls++;
-                map.put(tmp, map.get(tmp) - 1);
-            }
-        }
+        int[] count = new int[10];
 
-        for(int i = 0; i < length; i++) {
-            char tmp = guess.charAt(i);
-            if(tmp != secret.charAt(i) && map.getOrDefault(tmp, 0) != 0) {
-                cows++;
-                map.put(tmp, map.get(tmp) - 1);
+        for (int i = 0; i < len; i++) {
+            char s = secret.charAt(i);
+            char g = guess.charAt(i);
+            if (s == g) bulls++;
+            else {
+                if (count[s - '0'] < 0) {
+                    cows++;
+                }
+                count[s - '0']++;
+                
+                if (count[g - '0'] > 0) {
+                    cows++;
+                }
+                count[g - '0']--;
             }
         }
-        String res = bulls  + "A" + cows + "B";
-        return res;
+        return bulls + "A" + cows + "B";
     }
-}
+}
+
+