240. Search a 2D Matrix II

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 in ascending from left to right.
• Integers in each column are sorted in ascending from top to bottom.

For example,

Consider the following matrix:

[
  [1,   4,  7, 11, 15],
  [2,   5,  8, 12, 19],
  [3,   6,  9, 16, 22],
  [10, 13, 14, 17, 24],
  [18, 21, 23, 26, 30]
]

Given target = 5, return true.

Given target = 20, return false.

//查找二阶矩阵。

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        if(matrix.size()==0)
            return false;
            
        int rows = matrix.size();
        int columns = matrix[0].size();
        
        int i = rows-1;  
        int j = 0;  
        while(i>=0 && j<columns){  
            if (matrix[i][j] == target)  
                return true;  
            else if (matrix[i][j]<target)  
                j++;  
            else  
                i--;  
        }  
        return false; 
    }
};