LeetCode Remove Duplicate Letters

Description:

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.

Solution:

贪心问题，解决方案有很多，这里是用了stack来做。

遍历string，对于每个char，如果它比stack的peek小，并且该peek不是最后一次出现则循环出栈，然后如果该char没有在stack中出现，就入栈。

import java.util.*;public class Solution {public String removeDuplicateLetters(String s) {if (s == null)return "";int n = s.length();int last_pos[] = new int[26];for (int i = 0; i < n; i++)last_pos[s.charAt(i) - 'a'] = i;boolean exist[] = new boolean[26];boolean last_time[] = new boolean[26];Stack<Character> stack = new Stack<Character>();for (int i = 0; i < n; i++) {if (!exist[s.charAt(i) - 'a'])while (!stack.isEmpty() && stack.peek() >= s.charAt(i)&& !last_time[stack.peek() - 'a']) {exist[stack.peek() - 'a'] = false;stack.pop();}if (!exist[s.charAt(i) - 'a']) {stack.push(s.charAt(i));exist[s.charAt(i) - 'a'] = true;}if (last_pos[s.charAt(i) - 'a'] == i)last_time[s.charAt(i) - 'a'] = true;}String ret = "";while (!stack.isEmpty()) {ret = stack.pop() + ret;}return ret;}public boolean inPos(int k, int[] pos) {int n = pos.length;for (int i = 0; i < n; i++)if (pos[i] == k)return true;return false;}public static void main(String[] rags) {Solution s = new Solution();System.out.println(s.removeDuplicateLetters("abacb"));}}