763. Partition Labels

题目描述：

A string S of lowercase letters is given. We want to partition this string into as many parts as possible so that each letter appears in at most one part, and return a list of integers representing the size of these parts.

 

Example 1:

Input: S = "ababcbacadefegdehijhklij"
Output: [9,7,8]
Explanation:
The partition is "ababcbaca", "defegde", "hijhklij".
This is a partition so that each letter appears in at most one part.
A partition like "ababcbacadefegde", "hijhklij" is incorrect, because it splits S into less parts.

 

Note:

1. S will have length in range [1, 500].
2. S will consist of lowercase letters ('a' to 'z') only.

解题思路：

遍历S，找到S[i]在S中最后出现的位置j，此时遍历i到j中的每个元素S[index]，若S[index]在S中最后出现的位置大于j时，j等于S[index]在S中最后出现的位置，当index等于j时结束内循环。

预先记录每个字母在S最后出现的位置可以提高运算时间。

代码：

class Solution {
public:
    vector<int> partitionLabels(string S) {
        vector<int> res;
        for (int i = 0; i < S.size();) {
            size_t j = S.find_last_of(S[i]);
            if (i == j) {
                res.push_back(1);
                i += 1;
                continue;
            }
            for (int index = i + 1; index <= j; ++index) {
                size_t tmp = S.find_last_of(S[index]);
                if (tmp > j)
                    j = tmp;
            }
            res.push_back(j-i+1);
            i = j + 1;
        }
        return res;
    }
};