[leetcode]17. Letter Combinations of a Phone Number(Java)

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本文介绍了一种算法，用于根据电话按键对应的字母，生成一个电话号码的所有可能字母组合。提供了两种实现方式，一种使用递归回溯法，另一种采用迭代的方式。

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https://leetcode.com/problems/letter-combinations-of-a-phone-number/#/description

Given a digit string, return all possible letter combinations that the number could represent.

A mapping of digit to letters (just like on the telephone buttons) is given below.

Input:Digit string "23"
Output: ["ad", "ae", "af", "bd", "be", "bf", "cd", "ce", "cf"].

Note:
Although the above answer is in lexicographical order, your answer could be in any order you want.

package go.jacob.day718;

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


/**
 * 17. Letter Combinations of a Phone Number
 * @author Jacob
 *
 */
public class Demo2 {
	
	
	/*
	 * 牛客网:回溯法
	 * 解法二是leetcode的答案，题目稍有区别
	 */
	ArrayList<String> res=new ArrayList<String>();
	public ArrayList<String> letterCombinations(String digits) {
		if(digits==null||digits.length()==0){
			res.add("");
			return res;
			
		}
		
		String[] mapping={"0","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
		letterCombinations(digits,0,"",mapping);
		
		return res;
	}
	
	
	private void letterCombinations(String digits, int i, String string,String[] mapping) {
		if(i>=digits.length()){
			res.add(string);
			return;
		}
		String strs=mapping[Character.getNumericValue(digits.charAt(i))];
		
		for(char c:strs.toCharArray()){
			letterCombinations(digits,i+1,string+c,mapping);
		}
		
	}

	/*
	 * 解法二
	 * leetcode解法
	 */
	public List<String> letterCombinations_1(String digits) {
		LinkedList<String> res=new LinkedList<String>();
		if(digits==null||digits.length()==0)
			return res;
		//用字符串数组存储映射关系，不需要用map
		String[] mapping={"0","1","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};
		res.add("");
		for(int i=0;i<digits.length();i++){
			int x=Character.getNumericValue(digits.charAt(i));
			while(res.peek().length()==i){
				String temp=res.poll();
				for(char c:mapping[x].toCharArray()){
					res.add(temp+c);
				}
			}
		}
		return res;
	}
	
}