leetcode : Search a 2D Matrix

Posted 2020-09-02 notesbuddy

Write an efficient algorithm that searches for a value in an m x n matrix. This matrix has the following properties:

 

• Integers in each row are sorted from left to right.
• The first integer of each row is greater than the last integer of the previous row.

 

For example,

Consider the following matrix:

[
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]

Given target = 3, return true.

tag: 坐标转换 . 二分法 。binary search

 

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if(matrix == null || matrix.length == 0 || matrix[0] == null || matrix[0].length == 0){
            return false;
        }
        
        int rows = matrix.length;
        int cols = matrix[0].length;
        
        int length = rows * cols;
        
        int left = 0;
        int right = length - 1;
        
        while(left < right - 1){
            
            int mid = left + (right - left) / 2;
            int row = mid / cols;
            int col = mid % cols;
            
            if(matrix[row][col] == target){
                return true;
            }else if(matrix[row][col] < target){
                left = mid;
            }else{
                right = mid;
            }
            
        }
        
        if(matrix[left / cols][left % cols] == target){
            return true;
        }
        if(matrix[right / cols][right % cols] == target){
            return true;
        }
       
     return false;
		
    }
}