feat(leetcode): add No.1081

Author: mickey0524

5083.Occurrences-After-Bigram.java 1078.Occurrences-After-Bigram.java5083.Occurrences-After-Bigram.java renamed to 1078.Occurrences-After-Bigram.java

@@ -0,0 +1,46 @@
+// https://leetcode.com/problems/smallest-subsequence-of-distinct-characters/
+//
+// algorithms
+// Medium (38.69%)
+// Total Accepted: 1,667
+// Total Submissions: 4,311
+// beats 100.0% of java submissions
+
+class Solution {
+    public String smallestSubsequence(String text) {
+        int len = text.length();
+        LinkedList<Character> stack = new LinkedList<>();
+        
+        int[] lastIdx = new int[26];
+        boolean[] isUsed = new boolean[26];
+        Arrays.fill(lastIdx, -1);
+        for (int i = 0; i < len; i++) {
+            lastIdx[text.charAt(i) - 'a'] = i;
+        }
+        
+        for (int i = 0; i < len; i++) {
+            char ch = text.charAt(i);
+            while (!isUsed[ch - 'a'] && !stack.isEmpty()) {
+                char last = stack.peekLast();
+                if (ch <= last && lastIdx[last - 'a'] >= i) {
+                    stack.pollLast();
+                    isUsed[last - 'a'] = false;
+                } else {
+                    break;
+                }
+            }
+            
+            if (!isUsed[ch - 'a']) {
+                stack.offerLast(ch);
+                isUsed[ch - 'a'] = true;
+            }
+        }
+        
+        StringBuilder sb = new StringBuilder();
+        for (Character ch : stack) {
+            sb.append(ch);
+        }
+        
+        return sb.toString();
+    }
+}