Leetcode_17_电话号码的字母组合和_水题

最新推荐文章于 2025-02-12 02:30:00 发布

原创最新推荐文章于 2025-02-12 02:30:00 发布 · 113 阅读

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用string的简单尝试
回溯

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if (digits.length() == 0) {
            return ans;
        }
        Map<Character, String> map = new HashMap<Character, String>() {{
            put('2', "abc");
            put('3', "def");
            put('4', "ghi");
            put('5', "jkl");
            put('6', "mno");
            put('7', "pqrs");
            put('8', "tuv");
            put('9', "wxyz");
        }};
        backtrack(ans, digits, map, 0, new String());
        return ans;
    }

    public void backtrack(List<String> ans, String digits, Map<Character, String> map, int nowIndex, String sb) {
        if (nowIndex == digits.length()) {
            ans.add(sb.toString());
        } else {
            char nowChar = digits.charAt(nowIndex);
            String nowString = map.get(nowChar);
            for (int i = 0; i < nowString.length(); i++) {
                backtrack(ans, digits, map, nowIndex + 1, sb + nowString.charAt(i));
            }
        }
    }
}
//leetcode submit region end(Prohibit modification and deletion)

换了stringbuilder/stringbuffer速度直接起飞

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

//leetcode submit region begin(Prohibit modification and deletion)
class Solution {
    public List<String> letterCombinations(String digits) {
        List<String> ans = new ArrayList<>();
        if (digits.length() == 0) {
            return ans;
        }
        Map<Character, String> map = new HashMap<Character, String>() {{
            put('2', "abc");
            put('3', "def");
            put('4', "ghi");
            put('5', "jkl");
            put('6', "mno");
            put('7', "pqrs");
            put('8', "tuv");
            put('9', "wxyz");
        }};
        backtrack(ans, digits, map, 0, new StringBuilder());
        return ans;
    }

    public void backtrack(List<String> ans, String digits, Map<Character, String> map, int nowIndex, StringBuilder sb) {
        if (nowIndex == digits.length()) {
            ans.add(sb.toString());
        } else {
            char nowChar = digits.charAt(nowIndex);
            String nowString = map.get(nowChar);
            for (int i = 0; i < nowString.length(); i++) {
                backtrack(ans, digits, map, nowIndex + 1, sb.append(nowString.charAt(i)));
                sb.deleteCharAt(nowIndex);
            }
        }
    }
}
//leetcode submit region end(Prohibit modification and deletion)