leetcode 240 查询一个排序好的矩阵中是否存在target

首先题目如下所示：
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：从右上角开始往左下角找寻答案，如果大了就j–，如果小了就i++，这样子可以完全遍历矩阵，最后实现时间65ms，还可以了

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