Evaluate Nested Unions and Concatenations of Words

### Problem Evaluate the valid expression as a set of lowercase words. A letter denotes its one-letter word. Braces containing a comma-separated list of two or more expressions denote set union. Adjacent expressions concatenate every word from the first set with every word from the second. Braces may nest. Return each resulting word once, in lexicographic order. ```cpp class Solution { public: vector<string> braceExpansionII(string expression); }; ``` ### Examples **Example 1:** ```text Input: expression = "{x,y}{a,{b,c}}" Output: ["xa","xb","xc","ya","yb","yc"] Explanation: The second factor denotes a,b,c and combines with each first-factor letter. ``` **Example 2:** ```text Input: expression = "{{a,b},a{b,c},ab}" Output: ["a","ab","ac","b"] Explanation: The word ab arises twice but appears once in the sorted set. ``` ### Constraints - The expression length is between 1 and 60. - Characters are lowercase English letters, `{`, `}` or `,`. - The expression is valid under the stated grammar. ### Contract considerations This grammar allows nested expressions, unlike independent nonnested character-option groups. ```hint Read the boundary condition If two alternatives produce the same word, should it appear more than once in the returned list? ``` ### Boundary reminders This grammar allows nested expressions, unlike independent nonnested character-option groups.