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.
我们从右上角开始找，如果小于给定数值，那本行就直接排除，如果大于给定数值，本列就直接排除

var searchMatrix = function(matrix, target) {
    var row = matrix.length;
    if (row===0)
        return false;
    var col = matrix[0].length;
    if (col === 0)
        return false;
    var ir = 0;
    var ic = col-1;
    while (ir<row&&ic>=0) {
        if (matrix[ir][ic]>target)
            ic--;
        else if (matrix[ir][ic]<target)
            ir++;
        else 
            return true;
    }
    return false;
};