Interviews

[Google Onsite Winter 2018] Design Text Editor

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[Google] Design Text Editor (Doubly Linked List)

Build a text editor class with the following functions,

moveCursorLeft(),

moveCursorRight(),

insertCharacter(char) //insert the char right before cursor

backspace() //delete the char right before cursor

Follow-up

Implement undo() //undo the last edit. Can be called multiple times until all edits are cancelled.

All functions above should take O(1) time.

Example

( '|' denotes where the cursor locates. 'text' shows what's been written to the text editor. )

Start with empty text
text = "|"

insertCharacter('a')
text = "a|"

insertCharacter('b')
text = "ab|"

insertCharacter('c')
text = "abc|"

moveCursorLeft()
text = "ab|c"

moveCursorLeft()
text = "a|bc"

backspace()
text = "|bc"

moveCursorLeft()
text = "|bc" (nothing happens since cursor was on the leftmost position)

undo()
text = "a|bc"

undo()
text = "ab|c"

undo()
text = "abc|"

undo()
text = "ab|"

undo()
text = "a|"

Implementation

import java.util.Stack;
class CharNode {
    char ch;
    CharNode prev;
    CharNode next;

    public CharNode(char ch) { this.ch = ch; }
}

class Edit {
    String edition; //"DEL", "INS", "LEFT", "RIGHT"
    CharNode data;
    public Edit(String edition, CharNode cur) {
        data = cur;
        this.edition = edition;
    }
}

public class TextEditor {
    private Stack<Edit> undo_stack;
    private CharNode cursor;
    private CharNode end;

    public TextEditor() {
        undo_stack = new Stack();
        end = new CharNode('\0');
        cursor = end;
    }

    public void moveCursorLeft() {
        if(cursor.prev == null) return;
        cursor = cursor.prev;
        undo_stack.push(new Edit("RIGHT", null));
    }

    public void moveCursorRight() {
        if(cursor == end) return;
        cursor = cursor.next;
        undo_stack.push(new Edit("LEFT", null));
    }

    public void insertCharacter(char ch) {
        CharNode n = new CharNode(ch);
        insert(n);
        undo_stack.push(new Edit("DEL", null));
    }

    public void backspace() {
        if(cursor.prev == null) return;
        undo_stack.push(new Edit("INS", delete(cursor.prev)));
    }

    public void undo() {
        if(undo_stack.isEmpty()) return;
        Edit edit = undo_stack.pop();
        switch(edit.edition) {
            case "LEFT":
                cursor = cursor.prev; break;
            case "RIGHT":
                cursor = cursor.next; break;
            case "DEL":
                delete(cursor.prev); break;
            case "INS":
                insert(edit.data); break;
            default:
                return;
        }
    }

    public String toString() {
        StringBuilder text = new StringBuilder();
        CharNode n = end.prev;
        if(cursor == end) text.append('|');
        while(n != null) {
            text.insert(0, n.ch);
            if(cursor == n) text.insert(0, '|');
            n = n.prev;
        }
        return text.toString();
    }

    private void insert(CharNode node) {
        CharNode prev = cursor.prev;
        node.next = cursor;
        cursor.prev = node;
        if(prev != null) {
            prev.next = node;
            node.prev = prev;
        }
    }

    private CharNode delete(CharNode del) {
        if(del.prev != null) {
            del.prev.next = cursor;
        }
        cursor.prev = del.prev;
        return del;
    }
}


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