Search a 2D Matrix II

Posted 2020-06-09

Write an efficient algorithm that searches for a value in an m x n matrix, return the occurrence of it.

This matrix has the following properties:

• Integers in each row are sorted from left to right.
• Integers in each column are sorted from up to bottom.
• No duplicate integers in each row or column.

Example

Consider the following matrix:

[
  [1, 3, 5, 7],
  [2, 4, 7, 8],
  [3, 5, 9, 10]
]

Given target = 3, return 2.

Challenge

O(m+n) time and O(1) extra space

 

做法：

从每一行的最后一个开始比较，如果比target大，则去掉这一列。如果比target小，则去掉这一行。

public class Solution {
    /**
     * @param matrix: A list of lists of integers
     * @param: A number you want to search in the matrix
     * @return: An integer indicate the occurrence of target in the given matrix
     */
    public int searchMatrix(int[][] matrix, int target) {
        // write your code here
        if(matrix==null || matrix.length==0 || matrix[0].length==0) return 0;
        int count=0;
        int m=matrix.length;
        int n=matrix[0].length;
        
        int row=0;
        int col=n-1;
        
        while(row<m && col>=0)
        {
            if(matrix[row][col]>target)
            {
                col--;
            }
            else if(matrix[row][col]<target)
            {
                row++;
            }
            else
            {
                count++;
                row++;
                col--;
            }
        }
        
        return count;
    }
}