240. Search a 2D Matrix II

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        if(matrix.size()==0||matrix[0].size()==0)
            return false;
        int row=matrix.size();
        int col=matrix[0].size();
        int x=0, y=row-1;
        while(x<col&&y>-1){
            if(matrix[y][x]==target)
                return true;
            else if(matrix[y][x]>target)
                --y;
            else
                ++x;
        }
        return false;
    }
};

从左下角开始，当前值大于target就向上走，小于target就向右走