[Swift]LeetCode316. 去除重复字母 | Remove Duplicate Letters

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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"

---

给定一个仅包含小写字母的字符串，去除字符串中重复的字母，使得每个字母只出现一次。需保证返回结果的字典序最小（要求不能打乱其他字符的相对位置）。

示例 1:

输入: "bcabc"
输出: "abc"

示例 2:

输入: "cbacdcbc"
输出: "acdb"

---

32ms

class Solution {
    func removeDuplicateLetters(_ s: String) -> String {
        
        var counts = [Character: Int]()
        var used = [Character:Bool]()
        
        s.forEach {
            counts[$0, default: 0] += 1
        }
        
        var ans = [Character]()
        
        for ch in s {
            counts[ch, default:0] -= 1
            if used[ch, default: false] == true {
                continue
            }                      
            
            while let last = ans.last, ch < last, counts[last]! > 0 {
                used[last] = false
                ans.removeLast()           
            }
            ans.append(ch)
            used[ch] = true
        }

        return String(ans)
    }
}

---

36ms

extension Character {
    var ascii: UInt32? {   
        guard let first = unicodeScalars.first else {
            return nil
        }
        return first.isASCII == true ? first.value : nil    
    }
}

class Solution {
    class IndexedTree {
        var n: Int
        var arr: [Int]
        
        init(_ n: Int) {
            self.n = n
            arr = [Int](repeating: 0, count: n+1)
        }
        
        func insert(_ index: Int) -> Void {
            var x = index
            while x <= n {
                arr[x] += 1
                x = x + x&(-x)
            }
        }
        
        func query(_ index: Int) -> Int {
            var x = index
            var sum = 0
            while x > 0 {
                sum += arr[x]
                x = x&(x-1)
            }
            return sum
        }
    }
    

    func transIndex(_ n: Int, _ index: Int) -> Int {
        return n-index
    }
    
    func removeDuplicateLetters(_ s: String) -> String {
        let charArr = Array(s)
        var ans = ""
        
        let n = charArr.count
        if n == 0 {
            return ans
        }
        
        var counts = [Int](repeating: 0, count: 26)
        var indice = [Int?](repeating: nil, count: 26)
        var used = [Bool](repeating: false, count: 26)
        
        let indexedTree = IndexedTree(charArr.count)
        let aAsc = "a".first!.ascii!
        
        for (i, ch) in charArr.enumerated(){
            let ci = Int(ch.ascii!-aAsc)
            counts[ci] += 1
        }
        
        var que = [Int]()
        for (i, ch) in charArr.enumerated() {
            let ci = Int(ch.ascii!-aAsc)
            if counts[ci] == 0 {
                continue
            }


            
            if indice[ci] == nil {
                while let last = que.last {
                    if last > ci {
                        que.removeLast()
                        indice[last] = nil
                    } else {
                        break
                    }
                }
                
                indice[ci] = i
                que.append(ci)
            }
            
            counts[ci] -= 1
            if counts[ci] == 0 {
                while let ii = que.first, ii <= ci {
                    let index = indice[ii]!
                    ans += String(charArr[index])
                    indice[ii] = nil
                    counts[ii] = 0
                    que.removeFirst()
                }
            }

        }
    
        return ans
    }
}

---

52ms

class Solution {
    func removeDuplicateLetters(_ s: String) -> String {
        let sArr = Array(s)
        
        var counts = [Character : Int]()
        
        for c in sArr {
            if counts[c] == nil {
                counts[c] = 1
            }else {
                counts[c]! += 1
            }
        }
        
        var res = [Character]()
        
        for c in sArr {
            if !res.contains(c) {
                while !res.isEmpty && counts[res.last!] != 0 && res.last! > c  {
                    res.removeLast()
                }
                res.append(c)
                
            }
            counts[c]! -= 1
        }
        
        return String(res)
    }
}