74 - Search a 2D Matrix

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 from left to right.
• The first integer of each row is greater than the last integer of the previous row.

For example,

Consider the following matrix:

[
  [1,   3,  5,  7],
  [10, 11, 16, 20],
  [23, 30, 34, 50]
]

Given target = 3, return true.

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思路分析：

设置一个变量i表示行，变量j表示列，j初始指向向量尾。当matrix[i][j] < value，则 i++，否则 j--

代码如下：

class Solution {
public:
    bool searchMatrix(vector<vector<int> > &matrix, int target) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        int i = 0, j = matrix[0].size() - 1;
        
        while (i < matrix.size() && j >= 0)
        {
            if (target == matrix[i][j])
                return true;
            else if (target < matrix[i][j])
                j--;
            else
                i++;
        }
        
        return false;
    }
};