[LeetCode] 240. Search a 2D Matrix II 搜索一个二维矩阵 II
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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
.
74. Search a 2D Matrix 的变形,这题的矩阵特点是:每一行是按从左到右升序排列;每一列从上到下按升序排列。
解法:有特点的数是左下角和右上角的数。比如左下角的18开始,上面的数比它小,右边的数比它大,和目标数相比较,如果目标数大,就往右搜,如果目标数小,就往上搜。这样就可以判断目标数是否存在。或者从右上角15开始,左面的数比它小,下面的数比它大。
Python:
class Solution: def searchMatrix(self, matrix, target): m = len(matrix) if m == 0: return False n = len(matrix[0]) if n == 0: return False i, j = 0, n - 1 while i < m and j >= 0: if matrix[i][j] == target: return True elif matrix[i][j] > target: j -= 1 else: i += 1 return False
C++:
class Solution { public: bool searchMatrix(vector<vector<int> > &matrix, int target) { if (matrix.empty() || matrix[0].empty()) return false; if (target < matrix[0][0] || target > matrix.back().back()) return false; int x = matrix.size() - 1, y = 0; while (true) { if (matrix[x][y] > target) --x; else if (matrix[x][y] < target) ++y; else return true; if (x < 0 || y >= matrix[0].size()) break; } return false; } };
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[LeetCode] 74. Search a 2D Matrix 搜索一个二维矩阵
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