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Monday, August 7, 2017

[Leetcode] Search a 2D Matrix II, Solution

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.

[Thoughts]
双指针搜索。只不过因为数组的特殊排序,要从最右上角开始搜索,比如上例中的15.


[Code]
1:    bool searchMatrix(vector<vector<int>>& matrix, int target) {  
2:      int rows = matrix.size();  
3:      if(rows == 0) return false;  
4:      int cols = matrix[0].size();  
5:        
6:      int i = 0, j = cols-1;  
7:      while(j>=0 && i< rows) {  
8:        if(matrix[i][j] == target) return true;  
9:          
10:        if(matrix[i][j] < target) i++;  
11:        else j--;  
12:      }  
13:      return false;  
14:    }  


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