Given a string which contains only lowercase letters, remove duplicate letters so that every letter appear once and only once. You must make sure your result is the smallest in lexicographical order among all possible results.

Example 1:

Input: "bcabc"
Output: "abc"

Example 2:

Input: "cbacdcbc"
 

Output: "acdb"

給定一個僅包含小寫字母的字串，去除字串中重複的字母，使得每個字母只出現一次。需保證返回結果的字典序最小（要求不能打亂其他字元的相對位置）。

示例 1:

輸入: "bcabc"
輸出: "abc"

示例 2:

輸入: "cbacdcbc"
輸出: "acdb"

32ms

 1 class Solution {
 2     func removeDuplicateLetters(_ s: String) -> String {
 3         
 4         var counts = [Character: Int]()
 
 5         var used = [Character:Bool]()
 6         
 7         s.forEach {
 8             counts[$0, default: 0] += 1
 9         }
10         
11         var ans = [Character]()
12         
13         for ch in s {
14             counts[ch, default:0] -= 1
15             if used[ch, default: false
 
] == true {
16                 continue
17             }                      
18             
19             while let last = ans.last, ch < last, counts[last]! > 0 {
20                 used[last] = false
21                 ans.removeLast()           
22             }
23             ans.append(ch)
24             used[ch] = true
25         }
26 
27         return String(ans)
28     }
29 }

36ms

  1 extension Character {
  2     var ascii: UInt32? {   
  3         guard let first = unicodeScalars.first else {
  4             return nil
  5         }
  6         return first.isASCII == true ? first.value : nil    
  7     }
  8 }
  9 
 10 class Solution {
 11     class IndexedTree {
 12         var n: Int
 13         var arr: [Int]
 14         
 15         init(_ n: Int) {
 16             self.n = n
 17             arr = [Int](repeating: 0, count: n+1)
 18         }
 19         
 20         func insert(_ index: Int) -> Void {
 21             var x = index
 22             while x <= n {
 23                 arr[x] += 1
 24                 x = x + x&(-x)
 25             }
 26         }
 27         
 28         func query(_ index: Int) -> Int {
 29             var x = index
 30             var sum = 0
 31             while x > 0 {
 32                 sum += arr[x]
 33                 x = x&(x-1)
 34             }
 35             return sum
 36         }
 37     }
 38     
 39 
 40     func transIndex(_ n: Int, _ index: Int) -> Int {
 41         return n-index
 42     }
 43     
 44     func removeDuplicateLetters(_ s: String) -> String {
 45         let charArr = Array(s)
 46         var ans = ""
 47         
 48         let n = charArr.count
 49         if n == 0 {
 50             return ans
 51         }
 52         
 53         var counts = [Int](repeating: 0, count: 26)
 54         var indice = [Int?](repeating: nil, count: 26)
 55         var used = [Bool](repeating: false, count: 26)
 56         
 57         let indexedTree = IndexedTree(charArr.count)
 58         let aAsc = "a".first!.ascii!
 59         
 60         for (i, ch) in charArr.enumerated(){
 61             let ci = Int(ch.ascii!-aAsc)
 62             counts[ci] += 1
 63         }
 64         
 65         var que = [Int]()
 66         for (i, ch) in charArr.enumerated() {
 67             let ci = Int(ch.ascii!-aAsc)
 68             if counts[ci] == 0 {
 69                 continue
 70             }
 71 
 72 
 73             
 74             if indice[ci] == nil {
 75                 while let last = que.last {
 76                     if last > ci {
 77                         que.removeLast()
 78                         indice[last] = nil
 79                     } else {
 80                         break
 81                     }
 82                 }
 83                 
 84                 indice[ci] = i
 85                 que.append(ci)
 86             }
 87             
 88             counts[ci] -= 1
 89             if counts[ci] == 0 {
 90                 while let ii = que.first, ii <= ci {
 91                     let index = indice[ii]!
 92                     ans += String(charArr[index])
 93                     indice[ii] = nil
 94                     counts[ii] = 0
 95                     que.removeFirst()
 96                 }
 97             }
 98 
 99         }
100     
101         return ans
102     }
103 }

52ms

 1 class Solution {
 2     func removeDuplicateLetters(_ s: String) -> String {
 3         let sArr = Array(s)
 4         
 5         var counts = [Character : Int]()
 6         
 7         for c in sArr {
 8             if counts[c] == nil {
 9                 counts[c] = 1
10             }else {
11                 counts[c]! += 1
12             }
13         }
14         
15         var res = [Character]()
16         
17         for c in sArr {
18             if !res.contains(c) {
19                 while !res.isEmpty && counts[res.last!] != 0 && res.last! > c  {
20                     res.removeLast()
21                 }
22                 res.append(c)
23                 
24             }
25             counts[c]! -= 1
26         }
27         
28         return String(res)
29     }
30 }