Leetcode|中等|区间贪心|763. 划分字母区间（双指针+哈希表助力合并重叠区间）

class Solution {
public:
    vector<int> partitionLabels(string S) {
        if (S.empty()) return {};
        int size = S.size();
        vector<int> solution;
        vector<bool> used(size, false);
        int len = 0;
        for (int i = 0, first = 0, end = size - 1; i <= end; end--) {
            // 右指针只需要遍历到已确定区间外
            if (used[end]) {
                i++;
                end = size;
                continue;
            } // 若i走到片段边界，则添加该片段，并重置片段长度为0
            if (len && i >= first + len) {
                solution.emplace_back(len);
                first = i;
                len = 0;
            }
            // 合并区间
            if (!used[i] && S[i] == S[end]) {
                used[end] = true;
                len = max(len, end - first + 1);
                i++;  end = size;
            }// 段尾处理
            if (len + first == size) {
                solution.emplace_back(len);
                break;
            }
        }
        return solution;
    }
};

class Solution {
public:
    vector<int> partitionLabels(string S) {
        int hash[26] = {0};
        int size = S.size();
        vector<int> solution;
        // 1.构建最大字母索引哈希表
        for (int i = 0; i < size; i++)
            hash[S[i] - 'a'] = i;
        // 双指针包含片段
        int first = 0, end = 0;
        for (int i = 0; i < size; i++) {
            // 2.探索重叠区间，如果有则合并
            end = max(end, hash[S[i] - 'a']);
            if (i == end) { // 到达区间右边界则片段符合条件，添加到最终结果中
                solution.emplace_back(end - first + 1);
                first = i + 1;
            }
        }
        return solution;
    }
};