Medium：Search a 2D Matrix II

240. Search a 2D Matrix II Add to List

DescriptionHintsSubmissionsSolutionsTotal Accepted: 71813Total Submissions: 188647Difficulty: MediumContributor: LeetCodeWrite 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)。