240. Search a 2D Matrix II

问题描述：

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.

解题思路：

从最右上角开始，target>nums[0][n-1]，rows++;target<nums[0][n-1]，cols--.

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m=matrix.size();
        int n=matrix[0].size();
        if(matrix.size()==0) return false;
        int i=0,j=n-1;
        while(i<m&&j>=0)
        {
            if(matrix[i][j]==target)
                return true;
            else if(matrix[i][j]<target)
                i++;
            else j--;
        }
        return false;
    }
       
};