_17_LetterCombinationsofaPhoneNumber.java

package leetcode_1To300;

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

/**
 * 本代码来自 Cspiration，由 @Cspiration 提供
 * 题目来源：http://leetcode.com
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 * - 版权所有，转发请注明出处
 */
public class _17_LetterCombinationsofaPhoneNumber {
    /**
     * time : O(3^n)
     * space : O(n)
     */

    private String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};

    public List<String> letterCombinations(String digits) {
        List<String> res = new ArrayList<>();
        if (digits == null || digits.length() == 0) {
            return res;
        }
        helper(res, digits, "", 0);
        return res;
    }

    public void helper(List<String> res, String digits, String s, int index) {
        if (index == digits.length()) {
            res.add(s);
            return;
        }
        String letters = mapping[digits.charAt(index) - '0'];
        for (int i = 0; i < letters.length(); i++) {
            helper(res, digits, s + letters.charAt(i), index + 1);
        }
    }

    public List<String> letterCombinations2(String digits) {
        LinkedList<String> res = new LinkedList<>();
        if (digits == null || digits.length() == 0) {
            return res;
        }
        String[] mapping = new String[] {"0", "1", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        res.add("");
        for (int i = 0; i < digits.length(); i++) {
            int num = digits.charAt(i) - '0';
            while (res.peek().length() == i) {
                String t = res.remove();
                for (char s : mapping[num].toCharArray()) {
                    res.add(t + s);
                }
            }
        }
        return res;
    }

}