Medium：Search a 2D Matrix II

240. Search a 2D Matrix II Add to List

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Total Accepted: 71813
Total Submissions: 188647
Difficulty: Medium
Contributor: 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.
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.

Solution ★★★

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int row = 0;
        int col = matrix[0].size() - 1;

        while (row < matrix.size() && col >= 0) {
            if (matrix[row][col] == target)
                return true;
            else if (matrix[row][col] < target)
                ++row;
            else
                --col;
        }
        return false;
    }
};

解题思路：

• 先求出该matrix的行数和列数，若任意一个为零，则该矩阵为空，返回false；
• 初始化row，col，由于row为0，则后期若matrix[row][col]小则++row，大则–col。若随意–row, ++col容易引起越界；
• 时间复杂度为O(m+n)。