fd

Author: dingjikerbo

leetcode/solution/src/TopKFrequentElements.java

@@ -3,10 +3,39 @@
 public class TopKFrequentElements {
 
     /**
-     * 先统计每个元素次数，再用Priority排序
+     * 首选方法，时间复杂度O(n)，省去了排序
+     * 结尾注意list的size大于k了，要截一部分
      */
-    // 耗时46ms，最差复杂度O(nlgn)，当k<<n时为O(n)
     public List<Integer> topKFrequent(int[] nums, int k) {
+        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
+        int max = 0;
+        for (int n : nums) {
+            int count = map.getOrDefault(n, 0) + 1;
+            map.put(n, count);
+            max = Math.max(max, count);
+        }
+        List<Integer>[] lists = new LinkedList[max + 1];
+        for (int key : map.keySet()) {
+            int count = map.get(key);
+            if (lists[count] == null) {
+                lists[count] = new LinkedList<Integer>();
+            }
+            lists[count].add(key);
+        }
+        List<Integer> result = new LinkedList<Integer>();
+        for (int i = lists.length - 1; i >= 0 && result.size() < k; i--) {
+            if (lists[i] != null) {
+                result.addAll(lists[i]);
+            }
+        }
+        return result.subList(0, k);
+    }
+
+    /**
+     * 先统计每个元素次数，再用Priority排序
+     */
+    // 耗时46ms，最差复杂度O(nlgk)，当k<<n时为O(n)
+    public List<Integer> topKFrequent2(int[] nums, int k) {
         HashMap<Integer, Integer> map = new HashMap<>();
         for (int n : nums) {
             map.put(n, map.getOrDefault(n, 0) + 1);
@@ -30,7 +59,7 @@ public int compare(Integer o1, Integer o2) {
         return list;
     }
 
-    public List<Integer> topKFrequent2(int[] nums, int k) {
+    public List<Integer> topKFrequent3(int[] nums, int k) {
         Map<Integer, Integer> map = new HashMap<>();
         for(int n: nums){
             map.put(n, map.getOrDefault(n,0)+1);
@@ -52,31 +81,4 @@ public List<Integer> topKFrequent2(int[] nums, int k) {
         }
         return res;
     }
-
-    // 耗时23ms，时间复杂度O(n)，空间复杂度O(n)
-    // 这里有个问题，result的size可能大于k了
-    public List<Integer> topKFrequent3(int[] nums, int k) {
-        Map<Integer, Integer> map = new HashMap<Integer, Integer>();
-        int max = 0;
-        for (int n : nums) {
-            int count = map.getOrDefault(n, 0) + 1;
-            map.put(n, count);
-            max = Math.max(max, count);
-        }
-        List<Integer>[] lists = new LinkedList[max + 1];
-        for (int key : map.keySet()) {
-            int count = map.get(key);
-            if (lists[count] == null) {
-                lists[count] = new LinkedList<Integer>();
-            }
-            lists[count].add(key);
-        }
-        List<Integer> result = new LinkedList<Integer>();
-        for (int i = lists.length - 1; i >= 0 && result.size() < k; i--) {
-            if (lists[i] != null) {
-                result.addAll(lists[i]);
-            }
-        }
-        return result.subList(0, k);
-    }
 }

leetcode/src/Main.java

@@ -4,58 +4,34 @@ public class Main {
 
     public static class Solution {
 
-        public int ladderLength(String beginWord, String endWord, List<String> wordList) {
-            HashSet<String> words = new HashSet<>(wordList);
-            Queue<String> queue = new LinkedList<>();
-            queue.offer(beginWord);
-
-            Queue<String> next = new LinkedList<>();
-            int length = 1;
-
-            while (!queue.isEmpty()) {
-                String s = queue.poll();
-
-                if (s.equals(endWord)) {
-                    return length;
-                }
-
-                if (!words.isEmpty()) {
-                    StringBuilder sb = new StringBuilder(s);
-                    for (int i = 0; i < s.length(); i++) {
-                        for (char c = 'a'; c <= 'z'; c++) {
-                            if (c == s.charAt(i)) {
-                                continue;
-                            }
-
-                            sb.setCharAt(i, c);
-                            String st = sb.toString();
-
-                            if (words.contains(st)) {
-                                next.offer(st);
-                                words.remove(st);
-                            }
-                        }
-                        sb.setCharAt(i, s.charAt(i));
-                    }
+        public List<Integer> topKFrequent(int[] nums, int k) {
+            HashMap<Integer, Integer> map = new HashMap<>();
+            int max = 0;
+            for (int n : nums) {
+                int cnt = map.getOrDefault(n, 0) + 1;
+                map.put(n, cnt);
+                max = Math.max(max, cnt);
+            }
+            List<Integer>[] lists = new ArrayList[max + 1];
+            for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
+                int freq = entry.getValue();
+                if (lists[freq] == null) {
+                    lists[freq] = new ArrayList<>();
                 }
-
-                if (queue.isEmpty()) {
-                    queue.addAll(next);
-                    next.clear();
-                    length++;
+                lists[freq].add(entry.getKey());
+            }
+            List<Integer> result = new ArrayList<>();
+            for (int i = max; i >= 0 && result.size() <= k; i--) {
+                if (lists[i] != null) {
+                    result.addAll(lists[i]);
                 }
             }
-
-            return 0;
+            return result.subList(0, k);
         }
     }
 
 
     public static void main(String[] args) {
         Solution solution = new Solution();
-        int len = solution.ladderLength("hit", "cog", Arrays.asList(new String[]{
-                "hot", "dot", "dog", "lot", "log"
-        }));
-        System.out.println(len);
     }
 }