[LeetCode]Search a 2D Matrix

题目描述：(链接)

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.

解题思路：

二分查找

class Solution {
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int first = 0;
        int last = matrix.size() - 1;
        int len = matrix[0].size();
        while (first <= last) {
            int mid = first + (last - first) / 2;
            if (matrix[mid][0] <= target && target <= matrix[mid][len - 1] ) {
                return binarySearch(matrix[mid], target);
            } else if (matrix[mid][0] > target) {
                last = mid - 1;
            } else if (matrix[mid][len - 1] < target) {
                first = mid + 1;
            }
        }
        
        return false;
    }

private:
    bool binarySearch(vector<int>& vec, int target) {
        int first = 0;
        int last = vec.size() - 1;
        while (first <= last) {
            int mid= first + (last -first) / 2;
            if (vec[mid] == target) {
                return true;
            } else if (vec[mid] < target) {
                first = mid + 1;
            } else {
                last = mid - 1;
            }
        }
        
        return false;
    }
};