1003. Check If Word Is Valid After Substitutions Medium

Posted 2021-02-13 tornado549

参考：https://leetcode.com/problems/check-if-word-is-valid-after-substitutions/discuss/247626/JavaPythonC%2B%2B-Stack-Solution-O(N

方法1：使用c++自带的find函数和erase函数

class Solution {
public:
    bool isValid(string S) {
        while(!S.empty())
        {
            auto pos = S.find("abc");
            if(pos != string::npos)
                S.erase(pos,3);
            else
                return false;
        }
        return true;
    }
};

方法2：使用栈的思想

遇到 ‘c‘ 则检查栈内是否存在‘a‘, ‘b‘，并且要严格按照顺序排列

不合法则直接return false

若for循环终止后仍然未执行return，需要检查栈的大小，解决"aabcabc"这一类字符串

class Solution {
public:
    bool isValid(string S) {
        vector<char> stack;
        int n;
        for(char c : S)
        {
            n = stack.size();
            if(c == ‘c‘)
            {
                if(n < 2 || stack[n-1] != ‘b‘ || stack[n-2] != ‘a‘)
                    return false;
                stack.pop_back();
                stack.pop_back();
            }
            else
                stack.push_back(c);
        }
        return stack.size() == 0;
    }
};