240.Search a 2D Matrix II

Tags: Medium Binary Search Divide and Conquer

Links: https://leetcode.com/problems/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.

---

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

        return false;
    }
};

从左下角往右上角搜索，很常见的思路。