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.

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.

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

74. Search a 2D Matrix