LeetCode 240 Search a 2D Matrix II

Search a 2D Matrix II - LeetCode

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.

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.

本题关键在于右上角的特殊元素，如15，满足下面的数都比它大，左边的数都比它小。

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