LeetCode:Longest Increasing Path in a Matrix

Longest Increasing Path in a Matrix
Total Accepted: 15060 
Total Submissions: 46586 
Difficulty: Hard
Given an integer matrix, find the length of the longest increasing path.
From each cell, you can either move to four directions: left, right, up or down. You may NOT move diagonally or move 
outside of the boundary (i.e. wrap-around is not allowed).
Example 1:

nums = [
  [9,9,4],
  [6,6,8],
  [2,1,1]
]

Return  4 The longest increasing path is  [1, 2, 6, 9] .
Example 2:

nums = [
  [3,4,5],
  [3,2,6],
  [2,2,1]
]

Return  4 The longest increasing path is  [3, 4, 5, 6] . Moving diagonally is not allowed.
Credits: Special thanks to @dietpepsi for adding this problem and creating all test cases.
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Depth-first Search Topological Sort Memoization
考え方:
DFS.
java code:

public class Solution {
    
    public int longestIncreasingPath(int[][] matrix) {
        
        if(matrix == null || matrix.length == 0) return 0;
        int rows = matrix.length;
        int cols = matrix[0].length;
        
        int[][] maxLen = new int[rows][cols];
        
        int ans = 1;
        for(int i=0;i<rows;i++) {
            for(int j=0;j<cols;j++) {
                ans = Math.max(ans, DFS(matrix, maxLen, i, j, rows, cols));
            }
        }
        return ans;
    }
    
    //      
    
    private static final int[][] dirs = {{0,-1},{0,1},{-1,0},{1,0}}; //   
    
    private int DFS(int[][] matrix, int[][] maxLen, int i, int j, int rows, int cols) {
        
        if(maxLen[i][j] != 0) return maxLen[i][j];
        
        int max = 1;
        for(int[] dir : dirs) {
            int x = i + dir[0], y = j + dir[1];
            if(x < 0 || x >= rows || y < 0 || y >= cols) continue;
            if(matrix[i][j] <= matrix[x][y]) continue;
            
            max = Math.max(max, 1 + DFS(matrix, maxLen, x, y, rows, cols));
        }
        maxLen[i][j] = max;
        return max;
    }
    
    
}