Longest Increasing Path in a Matrix

class Solution {
    int maxLen(vector<vector<int>>& matrix, vector<vector<int>>& len, int r, int c) {
        if(len[r][c] != -1) {
            return len[r][c];
        }
        int max_len = 0;
        // Top
        if(r>0 && matrix[r][c]<matrix[r-1][c])
            max_len = max(max_len,maxLen(matrix,len,r-1,c));
        // Right
        if(c<matrix[0].size()-1 && matrix[r][c]<matrix[r][c+1])
            max_len = max(max_len,maxLen(matrix,len,r,c+1));
        // Down
        if(r<matrix.size()-1 && matrix[r][c]<matrix[r+1][c])
            max_len = max(max_len,maxLen(matrix,len,r+1,c));
        // Left
        if(c>0 && matrix[r][c]<matrix[r][c-1])
            max_len = max(max_len,maxLen(matrix,len,r,c-1));
        len[r][c] = 1+max_len;
        return len[r][c];
    }
public:
    int longestIncreasingPath(vector<vector<int>>& matrix) {
        int m = matrix.size();
        if(m<1)
            return 0;
        int n = matrix[0].size();
        vector<vector<int>> lenMat(m, vector<int>(n,-1));
        int max_length = -1;
        for(int i=0;i<m;++i) {
            for(int j=0;j<n;++j){
                max_length = max(max_length,maxLen(matrix,lenMat,i,j));
            }
        }
        return max_length;
    }
};