Search a 2D Matrix II

题目描述：

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：

从右上角row=0,col=cols-1开始查找target元素，如果matrix[row][col]=target返回true，如果matrix[row][col]>target，则令col--；如果matrix[row][col]<target，则令row++

AC代码如下：

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