74.Search a 2D Matrix

思路：

• 将二维数组看成一个一维数组，数组长度为m*n,对于位置x,它映射到matrix数组里面的位置为i/n,i%n，然后利用二分查找即可。

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        if(matrix.size() == 0) return false;
        int m = matrix.size();
        int n = matrix[0].size();
        
        int lo = 0;
        int hi = m*n-1;
        int mid,midi,midj;
        while(lo <= hi){
            mid = (lo + hi) / 2;
            midi = mid / n;
            midj = mid % n;
            if(matrix[midi][midj] == target) return true;
            else if(matrix[midi][midj] > target){
                hi = mid - 1;
            }else{
                lo = mid + 1;
            }
        }
        return false;
    }
};