亚马逊在线笔试题：SpecialSubstring和elimination game

import java.util.ArrayDeque;
import java.util.HashMap;

public class SpecialSubstring {
	/*
	 * Question 1 / 2 (Amazon Campus(8): SpecialSubstring)
	   find the longest substring that has at most 3 different characters.
	   for example:
	   		the special substring of "abbcddefghhh" is  "bbcdd".
	 */
	static String findsubstring(String a) {
		if(a==null) return null;
        HashMap<Integer,Integer> map = new HashMap<Integer, Integer>();
        int len = a.length(), start=0, end=0, sublen = 0;
        for(int i=0,j=0; i<len && j<len; j++){
        	int tail = a.charAt(j);
        	if(map.size()<3){//移动j
	        	if(map.containsKey(tail))
	        		map.put(tail, map.get(tail)+1);
	        	else
	        		map.put(tail, 1);
        	}else if(map.size()==3){
        		if(map.containsKey(tail))
	        		map.put(tail, map.get(tail)+1);
	        	else{//新的字符,移动i
	        		while(map.size()==3){
	        			int head = a.charAt(i++);
	        			map.put(head, map.get(head)-1);
	        			if(map.get(head)==0)
	        				map.remove(head);
	        		}
	        		map.put(tail, 1);
	        	}
        	}
        	//更新
        	if(j-i+1>sublen){
        		start = i; 
        		end = j; 
        		sublen=j-i+1;
        	}
        }
        if(sublen==0) return null;
        else return a.substring(start, end+1);
    }
	
	/*
	 * Question 2 / 2 (Amazon Campus(8): elimination game)
		have you played the elimination game before? When you hit a rectangle, other rectangles which are adjacent and in same color will be cleared. We could use a n*m matrix to indicate the game board. Numbers in the matrix indicate different color. Same number means same color.
		
		For example: this is a 5*4 game board:
		
		1 1 2 4
		2 3 3 3
		2 2 3 5
		3 3 3 3
		1 2 3 4
		
		The matrix index starts from 0. After you hit the rectangle at (3, 3), all the adjacent rectangle with number 3 will be cleared. We use number 0 to mark the cleared rectangle. The result will be: 
		
		1 1 2 4
		2 0 0 0
		2 2 0 5
		0 0 0 0
		1 2 0 4
		
		Put gravity into consideration, rectangles which locates on top of the 0-rect will fall down and the result will be like this:
		
		0 0 0 0
		1 0 0 0
		2 1 0 4
		2 2 0 5
		1 2 2 4
		
		The question is, given a m*n matrix and a (x,y) indicate which rectangle is hit, compute the result of this hit.
		
		Input:
		
		4 // the number of the columns
		
		5 // the number of the rows
		
		1 1 2 4 // each line of the metrix. Number are separated by space.
		2 3 3 3
		2 2 3 5
		3 3 3 3
		1 2 3 4
		
		3 // the x index of the hit position
		
		3 // the y index of the hit position
		
		Output:
		
		0 0 0 0 
		1 0 0 0
		2 1 0 4
		2 2 0 5
		1 2 2 4
		
		The output should be the matrix status line by line. Within each line, the numbers are separate by space.
	 */
	static String[] GetEliminationResult(String[] matrixString, int numOfColumn, int numOfRow, int hitXIndex, int hitYIndex) {
	    /*
	    matrixString_size is the number of strings stored in the matrixString. 
	    matrixString is an array of strings. Each string is a space seperated string like this "1 1 2 4". 
	    It is a row of the matrix
	    numOfColumn: Number of the columns in the matrix
	    numOfRow: Number of rows in the matrix
	    hitXIndex: the x index of the hit position
	    hitYIndex: the y index of the hit position
	    */
		int[][] matrix = new int[numOfRow][numOfColumn];
		for(int i=0; i<numOfRow; i++){
			String[] row = matrixString[i].split(" ");
			for(int j=0; j<numOfColumn; j++)
				matrix[i][j]=Integer.parseInt(row[j]);
		}
		class IntPair{
			int x;
			int y;
			IntPair(int x, int y){
				this.x = x;
				this.y = y;
			}
		}
		int color = matrix[hitXIndex][hitYIndex];
		if(color!=0){
			ArrayDeque<IntPair> queue = new ArrayDeque<IntPair>();
			matrix[hitXIndex][hitYIndex]=0;
			queue.add(new IntPair(hitXIndex, hitYIndex));
			while(!queue.isEmpty()){
				IntPair cell = queue.poll();
				int ax = cell.x, ay = cell.y;
				if(ax+1<numOfRow){
					if(matrix[ax+1][ay]==color){
						matrix[ax+1][ay]=0;
						queue.add(new IntPair(ax+1, ay));
					}
				}
				if(ax-1>=0){ 
					if(matrix[ax-1][ay]==color){
						matrix[ax-1][ay]=0;
						queue.add(new IntPair(ax-1, ay));
					}
				}
				if(ay+1<numOfColumn && matrix[ax][ay+1]==color){
					matrix[ax][ay+1]=0;
					queue.add(new IntPair(ax, ay+1));
				}
				if(ay-1>=0 && matrix[ax][ay-1]==color){
					matrix[ax][ay-1]=0;	
					queue.add(new IntPair(ax, ay-1));
				}
			}
		}
		//shift matrix
		for(int j=0; j<numOfColumn; j++){
			int z=numOfRow-1, i=z-1;
			while(true){
				while(z>=0 && matrix[z][j]!=0) z--;
				if(z < 0) break; i=z-1;
				while(i>=0 && matrix[i][j]==0) i--;
				if(i < 0) break;
				matrix[z--][j]=matrix[i][j];
				matrix[i--][j]=0;
			}
		}
		//return result
		String[] res = new String[numOfRow];
		for(int i=0; i<numOfRow; i++){
			String row = ""+matrix[i][0];
			for(int j=1; j<numOfColumn; j++)
				row += " "+matrix[i][j];
			res[i] = row;
		}
		return res;
	}
	public static void main(String[] args){
		String a = "abbcddefghhh";
		System.out.println(findsubstring(a));
		String[] matrix = new String[]{"1 1 2 4","2 3 3 3","2 2 3 5","3 3 3 3","1 2 3 4"};
		String[] res = GetEliminationResult(matrix, 4, 5, 3, 3);
		for(String row : res)
			System.out.println(row);
	}
}

bbcdd
0 0 0 0
1 0 0 0
2 1 0 4
2 2 0 5
1 2 2 4