240. Search a 2D Matrix II

• Difficulty: Medium

• Topics: Binary Search, Divide and Conquer

• Similar Questions:

  • Search a 2D Matrix

Problem:

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.

Solutions:

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m = matrix.size();
        if (m == 0) return false;
        int n = matrix[0].size();
        if (n == 0) return false;

        int i = m - 1;
        int j = 0;
        while (i >= 0 && j < n) {
            if (matrix[i][j] == target) return true;
            else if (matrix[i][j] < target) {
                ++j;
            } else {
                --i;
            }
        }

        return false;
    }
};