[leetcode]240. Search a 2D Matrix II

题目链接：https://leetcode.com/problems/search-a-2d-matrix-ii/#/description

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.

思路：从右上角开始遍历，如果matrix[ i ][ j ]<target, 说明该行所有的数都小于target,  则i++ 。如果matrix[ i ][ j ]>target,说明该列所有的数都大于target,则j--。

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