LeetCode: 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.

class Solution {public:    bool searchMatrix(vector<vector<int> > &matrix, int target) {        int m = matrix.size();        if(m == 0)            return false;        return binarySearch(0, m-1, target, matrix);    }private:    bool binarySearch(int start, int end, int target, vector<vector<int> > &matrix)    {        if(start > end)            return false;        int mid = (start+end)/2;        if(target <= matrix[mid].back() && target >= matrix[mid].front())        {            return binarySearch2(0, matrix[mid].size()-1, target, matrix[mid]);        }        else if(target < matrix[mid].front())        {             return binarySearch(start, mid-1, target, matrix);        }        else        {            return binarySearch(mid+1, end, target, matrix);        }        }    bool binarySearch2(int start, int end, int target, vector<int> &row)    {        if(start > end)            return false;        int mid = (start + end)/2;        if(target == row[mid])            return true;        else if(target < row[mid])            return binarySearch2(start, mid-1, target, row);        else        {            return binarySearch2(mid+1, end, target, row);        }            }};

Round 2:

class Solution {public:    bool searchMatrix(vector<vector<int> > &matrix, int target) {        int l = 0, r = matrix.size()-1;        while(l <= r)        {            int mid = (l+r)/2;            if(matrix[mid][0] == target)                return true;            else if(target >= matrix[mid][0] && target <= matrix[mid][matrix[0].size()-1])            {                l = 0, r = matrix[0].size()-1;                    while(l <= r)                    {                        int mid2 = (l+r) /2;                        if(matrix[mid][mid2] == target)                            return true;                        else if(matrix[mid][mid2] < target)                            l = mid2+1;                        else                            r = mid2-1;                    }                    return false;            }            else if(matrix[mid][0] > target)            {                r = mid-1;            }            else            {                l = mid+1;            }        }        return false;    }};