leetcode74 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 { //第一个算法书写简单，O(m+n),第二个算法书写稍复杂，O(log(m)+log(n))
public:
    bool searchMatrix(vector<vector<int>>& matrix, int target) {
        int m=matrix.size();
        if(!m)
            return false;
        int n=matrix[0].size();
/*        
        int i=0;
        while(i<m&&matrix[i][0]<=target)
        {
            i++;
        }
        i--;
        if(i<0)
            return false;
        for(int j=0;j<n;j++)
        {
            if(matrix[i][j]==target)
                return true;
        }
        return false;
*/
        
        int a=0,b=m-1;
        while(a<b)
        {
            int mid=a+(b-a)/2;
            int cur=matrix[mid][0];
            if(cur==target)
                return true;
            if(cur>target)
                b=mid;
            else
            {
                if(a==mid)
                {
                    if(matrix[b][0]>target)
                        b=a;
                    else
                        a=b;
                }
                else
                    a=mid;
            }
        }
        int i=a;
        a=0;b=n-1;
        while(a<=b)
        {
            int mid=a+(b-a)/2;
            int cur=matrix[i][mid];
            if(cur==target)
                return true;
            if(cur<target)
                a=mid+1;
            else
                b=mid-1;
        }
        return false;
    }
};