240. Search a 2D Matrix II

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：每次选右上角的元素拿来比较，如果大于target，左移一列，如果小于target则下移一行。

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