JZ19.java

// 19. 正则表达式匹配


public class Solution {
    public boolean match(char[] str, char[] pattern) {
        if (str == null || pattern == null) {
            return false;
        }
        int strIndex = 0, patternIndex = 0;
        return matchCore(str, strIndex, pattern, patternIndex);
    }

    public boolean matchCore(char[] str, int strIndex, char[] pattern, int patternIndex) {
        //有效性检验：str到尾，pattern到尾，匹配成功
        if (strIndex == str.length && patternIndex == pattern.length) {
            return true;
        }
        //pattern先到尾，匹配失败
        if (strIndex != str.length && patternIndex == pattern.length) {
            return false;
        }
        //模式第2个是*，且字符串第1个跟模式第1个匹配,分3种匹配模式；如不匹配，模式后移2位
        if (patternIndex + 1 < pattern.length && pattern[patternIndex + 1] == '*') {
            if (strIndex != str.length && (pattern[patternIndex] == str[strIndex] || pattern[patternIndex] == '.')) {
                return matchCore(str, strIndex, pattern, patternIndex + 2)      //模式后移2，视为x*匹配0个字符
                        || matchCore(str, strIndex + 1, pattern, patternIndex + 2)      //视为模式匹配1个字符
                        || matchCore(str, strIndex + 1, pattern, patternIndex);     //*匹配1个，再匹配str中的下一个
            } else {
                return matchCore(str, strIndex, pattern, patternIndex + 2);
            }
        }
        //模式第2个不是*，且字符串第1个跟模式第1个匹配，则都后移1位，否则直接返回false
        if (strIndex != str.length && (pattern[patternIndex] == str[strIndex] || pattern[patternIndex] == '.')) {
            return matchCore(str, strIndex + 1, pattern, patternIndex + 1);
        }
        return false;
    }
}