[LeetCode#74]Search a 2D Matrix

The problem:

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.

Solution1:

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        if (matrix == null || matrix.length == 0) //sterotype checking
            return false;
        int low = 0;
        int high = matrix.length - 1;
        int mid = -1;
        int row_index = -1;
        while (low <= high) { //search the row that may contain target
            mid = (low + high) / 2;
            if (matrix[mid][0] == target) {
                return true;
            } else if (matrix[mid][0] < target) {
                low = mid + 1;
            } else {
                high = mid - 1;
            }
        }
        /*a little skill: when exit while loop
        1. array[low] is the element just bigger than target;
        2. array[high] is th element just smaller than target.
        
        if (matrix[mid][0] > target) // the row that may contain target (mid + 1 is the possible to insert)
            row_index = mid - 1;
        else 
            row_index = mid; 
        */
        row_index = high;
        if (row_index < 0) //the target is even smaller than the smallest element in the matrix(first element)
            return false;
        low = 0;
        high = matrix[0].length - 1;
        mid = -1;
        while (low <= high) { //serach the target in the possible row
            mid = (low + high) / 2;
            if (matrix[row_index][mid] == target) {
                return true;
            } else if (matrix[row_index][mid] < target) {
                low = mid + 1;
            } else {
                high = mid - 1;
            }
        }
        return false;
    }
}

Solution2:

public class Solution {
    public boolean searchMatrix(int[][] matrix, int target) {
        int row_count = matrix.length;
        int column_count = matrix[0].length;
        if (matrix == null || matrix[0][0] > target || matrix[row_count-1][column_count-1] < target)
            return false;
        int i = 0;
        int j = column_count - 1;
        while (i <= row_count-1 && j >= 0) {
            if (matrix[i][j] == target)
                return true;
            else if (matrix[i][j] < target)
                i++;
            else
                j--;
        }
        return false;
    }
}