leetcode17:Letter Combinations of a Phone Number

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思路：采用深度优先遍历来做，需要通过变量记录输入的数字串的位置。

public class LetterCombinationsofaPhoneNumber17 {

	public static void main(String[] args) {
		String digits = "23";
		System.out.println(letterCombinations(digits));
	}

	public static List<String> letterCombinations(String digits) {
		List<String> list = new ArrayList<>();
		if (digits == null || digits.length() == 0)
			return list;
		String[] s = { "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz" };
		StringBuilder sb = new StringBuilder();
		dfs(digits, 0, s, sb, list);
		return list;
	}

	public static void dfs(String digits, int digitIdx, String[] s, StringBuilder sb, List<String> list) {
		if (digits.length() == sb.length()) {
			list.add(new String(sb.toString()));
			return;
		}
		//注意此处下标的位置
		String str = s[digits.charAt(digitIdx)-'2'];
		for (int i = 0; i < str.length(); i++) {
			sb.append(str.charAt(i));
			dfs(digits, digitIdx + 1, s, sb, list);
			sb.deleteCharAt(sb.length() - 1);
		}
	}
}

输出：[ad, ae, af, bd, be, bf, cd, ce, cf]