leetcode--17. 电话号码的字母组合--深度遍历优先

给定一个仅包含数字 2-9 的字符串，返回所有它能表示的字母组合。答案可以按 任意顺序 返回。

给出数字到字母的映射如下（与电话按键相同）。注意 1 不对应任何字母。

示例 1：

输入：digits = "23"
输出：["ad","ae","af","bd","be","bf","cd","ce","cf"]

示例 2：

输入：digits = ""
输出：[]

示例 3：

输入：digits = "2"
输出：["a","b","c"]

提示：

0 <= digits.length <= 4
digits[i] 是范围 ['2', '9'] 的一个数字。

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/letter-combinations-of-a-phone-number

解析:深度遍历优先一遍就可以得出结果。在数据结构讲树的部分时候，深度遍历优先是用于树的先序遍历。要将视野放的广一些，对于顺序的组合问题也是可以建成树的，并且是在树的叶子节点得到结果的。

#include<iostream>
#include<vector>
#include<map>
using namespace std;

vector<string> res;
map<string, vector<string>> alphaTable;
void createAlphaTable() {
	//pair<string, vector<string>> temp;
	alphaTable.insert(make_pair("2", vector<string>{"a", "b", "c"}));
	alphaTable.insert(make_pair("3", vector<string>{"d", "e", "f"}));
	alphaTable.insert(make_pair("4", vector<string>{"g", "h", "i"}));
	alphaTable.insert(make_pair("5", vector<string>{"j", "k", "l"}));
	alphaTable.insert(make_pair("6", vector<string>{"m", "n", "o"}));
	alphaTable.insert(make_pair("7", vector<string>{"p", "q", "r", "s"}));
	alphaTable.insert(make_pair("8", vector<string>{"t", "u", "v"}));
	alphaTable.insert(make_pair("9", vector<string>{"w", "x", "y", "z"}));
}

void dfs(string digits, string result) {
	if (digits == "" && result !="") {
		res.push_back(result);
	}
	string begin = digits.substr(0,1);
	for (auto i : alphaTable[begin]) {
		string temp = result + i;
		dfs(digits.substr(1), temp);
	}
}

int main() {
	string input;
	cin >> input;
	createAlphaTable();
	dfs(input, "");
	for (auto i : res)
		cout << i << " ";
}

class Solution {
public:
    map<string, vector<string>> alphaTable;
    vector<string> res;
    void createAlphaTable() {
        //pair<string, vector<string>> temp;
        alphaTable.insert(make_pair("2", vector<string>{"a", "b", "c"}));
        alphaTable.insert(make_pair("3", vector<string>{"d", "e", "f"}));
        alphaTable.insert(make_pair("4", vector<string>{"g", "h", "i"}));
        alphaTable.insert(make_pair("5", vector<string>{"j", "k", "l"}));
        alphaTable.insert(make_pair("6", vector<string>{"m", "n", "o"}));
        alphaTable.insert(make_pair("7", vector<string>{"p", "q", "r", "s"}));
        alphaTable.insert(make_pair("8", vector<string>{"t", "u", "v"}));
        alphaTable.insert(make_pair("9", vector<string>{"w", "x", "y", "z"}));
    }

    void dfs(string digits, string result) {
        if (digits == "" && result !="") {
            res.push_back(result);
        }
        string begin = digits.substr(0,1);
        for (auto i : alphaTable[begin]) {
            string temp = result + i;
            dfs(digits.substr(1), temp);
        }
    }

    vector<string> letterCombinations(string digits) {
        createAlphaTable();
        dfs(digits, "");
        return res;
    }
};