74. Search a 2D Matrix

class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int start = 0;
        int columnN= matrix[0].length;
        int end = matrix.length*columnN;
        
        while(start<end){
            int  mid = (start+end)/2;
            // important! use number of column or row!
            int i=mid/columnN;
            int j= mid%columnN;
            if(matrix[i][j] == target){
                return true;
            }else if(matrix[i][j]<target){
                start = mid+1;
            }else{
                end =mid;
            }
        }
        return false;
    }
}