316. Remove Duplicate Letters

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:

Given "bcabc"
Return "abc"

Given "cbacdcbc"
Return "acdb"

摘自

https://discuss.leetcode.com/topic/31404/a-short-o-n-recursive-greedy-solution

http://www.cnblogs.com/lasclocker/p/5037254.html

Given the string s, the greedy choice (i.e., the leftmost letter in the answer) is the smallest s[i], s.t.
the suffix s[i .. ] contains all the unique letters. (Note that, when there are more than one smallest s[i]'s, we choose the leftmost one. Why? Simply consider the example: "abcacb".)

After determining the greedy choice s[i], we get a new string s' from s by

1. removing all letters to the left of s[i],
2. removing all s[i]'s from s.

We then recursively solve the problem w.r.t. s'.

The runtime is O(26 * n) = O(n).

public class Solution {
    public String removeDuplicateLetters(String s) {
        int[] cnt = new int[26];
        int pos = 0; // the position for the smallest s[i]
        for (int i = 0; i < s.length(); i++) cnt[s.charAt(i) - 'a']++;
        for (int i = 0; i < s.length(); i++) {
            if (s.charAt(i) < s.charAt(pos)) pos = i;
            if (--cnt[s.charAt(i) - 'a'] == 0) break; //保证字符是全的，例如"bcabc",到a位置就终止了，也保证了最小字符前面的可以删，因为包括当前操作的位置，往后至少所有所有字符还会有一份
        }
        return s.length() == 0 ? "" : s.charAt(pos) + removeDuplicateLetters(s.substring(pos + 1).replaceAll("" + s.charAt(pos), ""));//当前位置的字符+迭代删除以后的字符中去掉当前字符
    }
}

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给所有单词计数，每次检查到一个位置，就给这个位置上的字母计数值-1，维护一个栈，当前元素如果比栈顶的字典序小，在计数值不为0的情况下（表明如果现在把这个字母从栈顶弹出，后面还存在当前被出栈的字母）就出栈直到栈空或者当前元素比栈顶字母字典序大，这样处理一遍就得到所求的。栈可以用StringBuilder代替

public String removeDuplicateLetters3(String s) {
	        
	        int[] res = new int[26]; // will contain number of occurences of character (i+'a')
	        boolean[] visited = new boolean[26]; // will contain if character ('a' + i) is present in current result Stack
	        char[] ch = s.toCharArray();
	        for(char c : ch){  // count number of occurences of character 
	            res[c-'a']++;
	        }
	        StringBuilder sb = new StringBuilder();; // answer stack
	        int index;
	        for(char c : ch){ 
	            index = c - 'a';
	            res[index]--;   // decrement number of characters remaining in the string to be analysed
	            if(visited[index]) // if character is already present in stack, dont bother
	                continue;
	            // if current character is smaller than last character in stack which occurs later in the string again
	            // it can be removed and  added later e.g stack = bc remaining string abc then a can pop b and then c
	            while( (sb.length() > 0) && c < sb.charAt(sb.length()-1) && res[sb.charAt(sb.length()-1)-'a']!=0){ 
	                visited[sb.charAt(sb.length()-1) - 'a'] = false;
	                sb.deleteCharAt(sb.length()-1);
	            }
	            sb.append(c); // add current character and mark it as visited
	            visited[index] = true;
	        }
	        
	        return sb.toString();
	    
	    }

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分析：通过观察每个字母下标的规律，以"cbacdcbc"为例，

第一步，计算下标(countIndex)：

a  => 2,  

b  => 1, 6

c  => 0, 3, 5, 7

d  => 4

第二步，寻找符合条件的字母(findLetter)：

如果一个字母的第一个下标小于其后每个字母的最后一个下标，则该字母符合条件。比如 a 中的 2 小于 b 的 6，c 的 7，d 的 4，则 a 中的2符合条件。

第三步，删除前面的下标(removeIndex)：

不防设第二步中符合条件的字母为X，则删除X下标之前的所有下标，并删除X。比如X为a，X下标为2，则删除 b 中的 1，c 中的 0，并把 a 删除。

重复第二步，第三步，直到集合为空。

以 "cbacdcbc" 为例：

b  => 6

c  => 3, 5, 7

d  => 4

b  => 6



d  => 4

上述变换解释：虽然 6 < 7，但 6 却 > 4， 故 b 不符合，而 c 中的 3 小于其后的 4，故 c 符合。 

b  => 6

故最后生成 a -> c -> d -> b，即 acdb.

代码如下：

    public String removeDuplicateLetters(String s) {
        StringBuilder res = new StringBuilder();
        HashMap> hm = new HashMap>();
        ArrayList reference = new ArrayList();
        countIndex(s, hm, reference);
        Collections.sort(reference);
        while (!reference.isEmpty()) {
            int lettIndex = findLetter(s, res, hm, reference);
            removeIndex(lettIndex, hm, reference);
        }
        return res.toString();
    }
    
    private void countIndex(String s, HashMap> hm, ArrayList reference) {
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);
            if (! hm.containsKey(ch)) {
                ArrayList tmp = new ArrayList();
                tmp.add(i);
                hm.put(ch, tmp);
                reference.add(ch);
            } else {
                hm.get(ch).add(i);
            }
        }
    }
    
    private int findLetter(String s, StringBuilder res, HashMap> hm, ArrayList reference) {
        int m = 0;
        for (int i = 0; i < reference.size(); i++) {
            m = hm.get(reference.get(i)).get(0);
            int j = i+1;
            for (; j < reference.size(); j++) {
                ArrayList tmp = hm.get(reference.get(j));
                if (m > tmp.get(tmp.size()-1)) {
                    break;
                }
            }
            if (j == reference.size()) {
                res.append(reference.get(i));
                reference.remove(i);
                break;
            }
        }
        return m;
    }
    
    private void removeIndex(int lett, HashMap> hm, ArrayList reference) {
        for (int i = 0; i < reference.size(); i++) {
            ArrayList tmp = hm.get(reference.get(i));
            while (!tmp.isEmpty() && tmp.get(0) < lett) {
                tmp.remove(0);
            }
        }
    }