240. Search a 2D Matrix II
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1.题目描述
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.
一开始以为是要实现将一个二维矩阵按以上规则进行排序,后来才发现只需要在矩阵中寻找相应的数字是否存在即可。
2.算法
第一种:最直接的暴力解法就是直接遍历二维数组,时间是O(m*n)
第二种:因为该二维矩阵有一定的规律,从第一行开始比较,每次从最后一列往左,如果该行的最后一列的数字比target要小的话,则i++,如果比target大的话,则j-;该算法相比第一种算法就要好很多,时间复杂度是O(n+m)
3.代码实现
第一种算法:
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; for (int i = 0; i < m; ++i) { for (int j = 0; j < n; ++j) { if (matrix[i][j] == target) { cout << matrix[i][j] << "here" << endl; return true; } } } return false; }
第二种算法:
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 = 0, j = n - 1; while (i < m && j >= 0) { if (target == matrix[i][j]) { return true; } else if (target > matrix[i][j]) { i++; } else { j--; } } return false;}
main函数,用于测试
#include <iostream>#include <vector>using namespace std;bool searchMatrix(vector<vector<int> >& matrix, int target);int main() { int m, n, value; cout << true << endl; cout << "row number: "; cin >> m; cout << "column number: "; cin >> n; vector<vector<int> > data; vector<int> rowData; for (int i = 0; i < m; ++i) { rowData.clear(); for (int j = 0; j < n; ++j) { cin >> value; rowData.push_back(value); } data.push_back(rowData); } int target; cout << "target: "; while (cin >> target) cout << searchMatrix(data, target) << endl; return 0;}
4.小结
虽然在leetcode上面O(n^2)的算法常常可以顺利的通过,但是第二种算法的实现思路还是很值得借鉴。
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240. Search a 2D Matrix II
- 240.Search a 2D Matrix II
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