真是一道好题。

一眼看过去，一想，是bfs; 然后因为是combination类:典型的dfs.

bfs:

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public List<String> letterCombinations(String digits) {
        String[] mapping = new String[]{"", "", "abc", "def", "ghi", "jkl", "mno", "pqrs", "tuv", "wxyz"};
        LinkedList<String> res = new LinkedList<>();
        if (digits.length() == 0) return res;
        res.add("");
        
        while (res.peek().length() != digits.length()) {
            String tmp = res.poll();
            String add = mapping[digits.charAt(tmp.length()) - '0'];
            for (char c : add.toCharArray()) {
                res.add(tmp + c);
            }
        }
        
        return res;
    }

res.add("")是为了启动while,如果没有就直接return了

dfs:

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

public List<String> letterCombinations(String digits) {
    LinkedList<String> res = new LinkedList<>();
    combination("", digits, 0, res);
    return res;
}

public void combination(String prefix, String digits, int i, LinkedList<String> res) {
    if (digits == null || digits.length() == 0) return;
    if (prefix.length() >= digits.length()) {
        res.add(prefix);
        return;
    }
    String tmp = mapping[digits.charAt(i) - '0'];
    for (char c : tmp.toCharArray()) {
        combination(prefix+c, digits, i+1, res);
    }
}

复习static修饰成员变量: 成员变量属于类对象，而不是哪一个具体的实例对象

复习LinkedList和纯粹List方法的不同: 有的对象声明为List类型, 即使多态右边new对象是LinkedList,还是会因为左边本身的类型是List而不能用那个方法。

List: add, get, isEmpty, remove, size

LinkedList: add, addFirst, addLast, getFirst, getLast, offer, peek, poll, remove, push, size (也就是Queue的方法的几个方法都是只有LinkedList独有而List没有, 毕竟LinkedList是Queue的实现类)