[LeetCode] Search a 2D Matrix

Thought:
First find the row and then find the column.

Code:
public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if (matrix.length == 0) return false;
       
        int m = matrix.length, n = matrix[0].length;
       
        int[] a = new int[m];
        for (int i = 0; i < m; i++) a[i] = matrix[i][0];
       
        int row = helper(a, target);
        if (row == -1) return false;
        int col = helper(matrix[row], target);
        return matrix[row][col] == target;
    }
    public int helper (int[] a, int x) {
       
        int start = 0, end = a.length - 1, mid = (a.length - 1) / 2;
        int ret = -1;

        while (start <= end) {
            if (a[mid] > x) {
                end = mid - 1;
                mid = (start + end) / 2;
            }else {
                start = mid + 1;
                ret = mid;
                mid = (start + end) / 2;
            }
        }

        return ret;
    }
}