71. Simplify Path

原题：

Given an absolute path for a file (Unix-style), simplify it.

For example,
path = "/home/", => "/home"
path = "/a/./b/../../c/", => "/c"

click to show corner cases.

Corner Cases:

• Did you consider the case where path = "/../"?
  In this case, you should return "/".
• Another corner case is the path might contain multiple slashes '/' together, such as"/home//foo/".
  In this case, you should ignore redundant slashes and return "/home/foo".

思考过程：

一开始分很多情况进行操作，后来干脆用了自动机，不同状态进行不同处理。用ArrayList实现一个栈存储已经遍历过的目录，遇到"/../"就出栈。

几个终态含义：4表示/../；6表示/./；7表示正常目录。

AC代码：

ArrayList<String> stack = new ArrayList<>();    int stackTop = -1;    String currentString = "/";    int state = 1;//标识自动机状态    public String simplifyPath(String path) {        String ret = "";        int len = path.length();        for (int i = 0;i < len;i++){            char c = path.charAt(i);            if (c != '/') currentString += c;            int[] reslutState = new int[3];            switch (state){                case 1:{                    reslutState[0] = 1;                    reslutState[1] = 2;                    reslutState[2] = 5;                    break;                }                case 2:{                    reslutState[0] = 6;                    reslutState[1] = 3;                    reslutState[2] = 5;                    break;                }                case 3:{                    reslutState[0] = 4;                    reslutState[1] = 5;//"..."算是路径                    reslutState[2] = 5;                    break;                }                case 5:{                    reslutState[0] = 7;                    reslutState[1] = 5;                    reslutState[2] = 5;                    }            }            if (state != 4 && state != 7) state = changeState(state,c,reslutState);//非终点集转换状态即可            if (state == 4 || state == 7 || state == 6)manageEndpoint(state);        }        if (currentString.charAt(currentString.length() - 1) != '/') {//处理最后一段不以'/'结尾的路径            if (state == 5)                push();            if (state == 3)                pop();        }        if (stackTop == -1) return "/";        for (int i = 0;i <= stackTop;i++) ret += stack.get(i);        return ret;    }    /*    出栈函数     */    public void pop(){        if (stackTop > -1) {            stack.remove(stackTop);            stackTop--;//遇到".."的情况要返回上级目录，也就是出栈        }    }    /*    入栈函数     */    public void push(){        String s = currentString;        stack.add(s);        stackTop++;    }    public void manageEndpoint(int state){        switch (state){//终点集要进行处理            case 4:{                pop();                break;            }            case 7 :                push();//放入此段路径        }        this.state = 1;//处理完此目录路径，恢复初始状态        currentString = "/";//当前字符串也恢复初始状态    }    public int changeState(int state,char c,int[] resultState){//用于切换自动机状态        switch (c){            case '/':{                state = resultState[0];                break;            }            case '.':{                state = resultState[1];                break;            }            default:{                state = resultState[2];            }        }        return state;    }