[LeetCode]Simplify Path

class Solution {
//1. skip // and ./
//2. when meet ../ pop
//3. keep record of word between /word/
public:
	string simplifyPath(string path) {
		stack<int> sk;
		string p;

		path.append("/"); // normalize the back slash in the end, so we can keep record of the last word
		sk.push(0);
		int s=0;
		for (int i=1; i<path.size(); i++) {
			if (path[i] == '/') {
				if (path[i-1] == '/'
					|| path[i-1] == '.' && (i-2 < 0 || path[i-2] != '.')) {
						// skip double //  or /./
						s = i;
				}
				else if (path[i-1] == '.' && i-2 >= 0 && path[i-2] == '.') {
					s = i;
					if (sk.top() == 0) continue;
					// popping one part
					int e = sk.top(); sk.pop();
					p.erase(sk.top(), e-sk.top());
				}
				else {
					// add new part
					p.append(path, s, i-s);
					sk.push(p.size());
					s = i;
				}
			}
		}

		return p.size() == 0 ? "/" : p;
	}
};

second time

class Solution {
public:
    bool checkValidFile(string str)
    {
        for(int i = 0; i < str.size(); ++i)
            if((str[i] >= 'a' && str[i] <= 'z') 
            || (str[i] >= 'A' && str[i] <= 'Z')) return true;
        return false;
    }
    string simplifyPath(string path) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<string> files;
        int start = 0;
        while(start < path.size())
        {
            int end = start;
            while(end < path.size() && path[end] != '/') end++;//... case: "/a/.." ?
            string tmp = path.substr(start, end-start);
            //if(tmp.size() == 2 && tmp[0] == '.' && tmp[1] == '.' && !files.empty())  files.pop();
            if(tmp == ".." && !files.empty())  files.pop();
            else if(checkValidFile(tmp)) files.push(tmp);
            start = end+1;
        }
        
        string ans;
        stack<string> fileReverse;
        while(!files.empty()) fileReverse.push(files.top()), files.pop();
        while(!fileReverse.empty())
        {
            ans += "/";
            ans += fileReverse.top();
            fileReverse.pop();
        }
        if(ans.size() == 0) ans = "/";
        return ans;
    }
};