Maximal Rectangle

地址:https://oj.leetcode.com/problems/maximal-rectangle/

Given a 2D binary matrix filled with 0's and 1's, find the largest rectangle containing all ones and return its area.

首先吐槽下这个题目,不知道是我英语理解还是题目本身存在让人误解地方,我理解题意是最大的矩形包括所有1元素,按着这个理解我很开心的写了如下代码:代码思路很简单,类似杨氏矩阵中用到的思想:http://blog.csdn.net/huruzun/article/details/28420545

public class Solution {
        private char[][] matrix ;
	public int maximalRectangle(char[][] matrix) {
		if(matrix.length == 0){
			return 0;
		}
		int row = matrix.length;
		int col = matrix[0].length;
		int ans = 0;
		this.matrix = matrix;
		Point start = new Point();
		Point end = new Point(row-1,col-1);
		while(true){
			if(isAllZeroDown(start, row)){
				start.y = start.y+1 >=col ? col-1:start.y+1;
				if(start.equals(end)){
					return 0;
				}
			}
			if(isAllZeroLeft(start, col)){
				start.x = start.x+1 >=row ? row-1:start.x+1;
				if(start.equals(end)){
					return 0;
				}
			}
			
			if(!isAllZeroDown(start, row) && !isAllZeroLeft(start, col)){
				break;
			}
		}
		
		while(true){
			if(isAllZeroRight(end)){
				end.y = end.y-1>= 0 ? end.y-1:0;
			}
			if(isAllZeroUp(end)){
				end.x = end.x-1>= 0 ? end.x-1:0;
			}
			if(!isAllZeroRight(end) && !isAllZeroUp(end)){
				break;
			}
		}
		ans = Math.abs((end.x-start.x+1)*(end.y - start.y+1));
		return ans;
	}
	
	boolean isAllZeroDown(Point p,int len){
		int j = p.y;
		for(int i=p.x;i<len;i++){
			if(matrix[i][j]=='1'){
				return false;
			}
		}
		return true;
	}
	boolean isAllZeroLeft(Point p,int len){
		int i = p.x;
		for(int j=p.y;j<len;j++){
			if(matrix[i][j]=='1'){
				return false;
			}
		}
		return true;
	}
	boolean isAllZeroUp(Point p){
		int i = p.x;
		for(int j=p.y;j>=0;j--){
			if(matrix[i][j]=='1'){
				return false;
			}
		}
		return true;
	}
	boolean isAllZeroRight(Point p){
		int j = p.y;
		for(int i=p.x;i>=0;i--){
			if(matrix[i][j]=='1'){
				return false;
			}
		}
		return true;
	}
}


提交之后始终不对,然后开始网上搜索,才发现题目正确意思是:

找一个最大矩阵,里面全部是1。

例如下图:

Maximal Rectangle_第1张图片

根据这个图转换可以发现问题,看下图:

Maximal Rectangle_第2张图片

 

转到这个图就会发现,这就是:http://blog.csdn.net/huruzun/article/details/39717501里面说到的相同问题。

java代码:

public class Solution {
    	public int largestRectangleArea(int[] height) {
		int maxarea = 0;
		Stack<Integer> sta = new Stack<>();
		int top ;
		int top_area;
		int i = 0;
		while(i<height.length){
			if(sta.isEmpty() || height[sta.peek()]<=height[i] ){
				sta.push(i++);
			}else{
				top = sta.pop();
				top_area = height[top] * (sta.isEmpty()? i:i-sta.peek()-1);
				if(top_area>maxarea){
					maxarea = top_area;
				}
			}
		}
		while(!sta.isEmpty()){
			top = sta.pop();
			top_area = height[top] * (sta.isEmpty()? i:i-sta.peek()-1);
			if(top_area>maxarea){
				maxarea = top_area;
			}
		}
		return maxarea;
	}
	
	public int maximalRectangle(char[][] matrix){
		if(matrix.length == 0){
			return 0;
		}
		int row = matrix.length;
		int col = matrix[0].length;
		int [][]dp = new int[row][];
		for(int i=0;i<row;i++){
			dp[i] = new int[col];
		}
		
		for(int j=0;j<col;j++){
			if(matrix[0][j]=='1'){
				dp[0][j] = 1;
			}
		}
		for(int j=0;j<col;j++){
			for(int i=1;i<row;i++){
				if(matrix[i][j] == '1'){
					dp[i][j] = dp[i-1][j]+1;
				}
			}
		}
		int maxarea = 0;
		for(int i=0;i<row;i++){
			int temp = largestRectangleArea(dp[i]);
			if(temp>maxarea){
				maxarea = temp;
			}
		}
		return maxarea;
	}
}


 

 

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