LetterCasePermutation*

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Given a string S, we can transform every letter individually to be lowercase or uppercase to create another string.  Return a list of all possible strings we could create.

Examples:
Input: S = "a1b2"
Output: ["a1b2", "a1B2", "A1b2", "A1B2"]

Input: S = "3z4"
Output: ["3z4", "3Z4"]

Input: S = "12345"
Output: ["12345"]
Note:

S will be a string with length between 1 and 12.
S will consist only of letters or digits.
在真实的面试中遇到过这道题？

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class Solution {
public:
	vector<string> letterCasePermutation(string S) {
		vector<string> res;
		backTracking(0, res, S);
		return res;
	}
	void backTracking(int start, vector<string>& res, string s) {
		if (start == s.size()) {
			res.push_back(s);
			return;
		}

		if (s[start] >= '0'&&s[start] <= '9') {
			backTracking(start + 1, res, s);
			return;
		}

		s[start] = tolower(s[start]);
		backTracking(start + 1, res, s);

		s[start] = toupper(s[start]);
		backTracking(start + 1, res, s);
	}
};