Maximal Square

Given a 2D binary matrix filled with 0's and 1's, find the largest square containing all 1's and return its area.

For example, given the following matrix:

1 0 1 0 0
1 0 1 1 1
1 1 1 1 1
1 0 0 1 0

Return 4.

class Solution {
public:
    int maximalSquare(vector<vector<char>>& matrix) {
        int n=matrix.size();
        if(n<1) return 0;
        int m=matrix[0].size();
        if(m<1) return 0;
        vector<vector<int>> size(n,vector<int>(m,0));
        int maxnum=0;
        int i;
        for(i=0;i<n;i++)
        {
            if(matrix[i][0]=='1')
            {
                size[i][0]=1;
                maxnum=1;
            }
        }
        for(i=0;i<m;i++)
        {
            if(matrix[0][i]=='1')
            {
                size[0][i]=1;
                maxnum=1;
            }
        }
        
        int j;
        for(i=1;i<n;i++)
        {
            for(j=1;j<m;j++)
            {
                if(matrix[i][j]=='1')
                {
                    size[i][j]=min(size[i-1][j-1],min(size[i-1][j],size[i][j-1]))+1;
                    if(maxnum<size[i][j])
                       maxnum=size[i][j];
                }
                else
                    size[i][j]=0;
            }
        }
        return maxnum*maxnum;
    }
};