74. Search a 2D Matrix

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
.
这个个是标准的利用二分法搜索的题目，先二分搜索出行，再在行里进行进一步的二分搜索：

var searchMatrix = function(matrix, target) {
    if (target<matrix[0][0])
        return false;
    var row = matrix.length;
    var col = matrix[0].length;
    if (target>matrix[row-1][col-1])
        return false;
    var top = 0;
    var bottom = row-1;
    while (top<=bottom) {
        var mid = top + parseInt((bottom-top)/2);
        if (target<matrix[mid][0])
            bottom = mid-1;
        else if (target>matrix[mid][col-1])
            top = mid+1;
        else {
            var left = 0;
            var right = col-1;
            while(left<=right) {
                var center = left + parseInt((right-left)/2);
                if (target < matrix[mid][center])
                    right = center -1;
                else if (target > matrix[mid][center])
                    left = center +1;
                else 
                    return true;
            }
            break;
        }
    }
    return false;
};