Longest Increasing Path in a Matrix

给一个2d数组, 找出其中最长的递增路径. 用dp作为一个memo. 然后用longest去记录路径的最长值. 因为题目不要求返回路径, 所以直接返回值就可以.

public class Solution {
    public int longestIncreasingPath(int[][] matrix) {
        if(matrix == null || matrix.length == 0)
            return 0;
        int n = matrix.length;
        int m = matrix[0].length;
        int[][]dp = new int[n][m];
        int longest = 1;
        for(int i = 0 ; i < n; i++) {
            for(int j = 0; j < m; j++) {
                longest = Math.max(longest, dfs(matrix,i,j,dp));
            }
        }
        return longest;
    }
    
    public int dfs(int[][]matrix, int i , int j, int[][] dp) {
        if(dp[i][j] > 0)
            return dp[i][j];
        int longest = 1;
        int cur = matrix[i][j];
        if(i-1>=0 && matrix[i-1][j] > cur) //up
            longest = Math.max(longest, dfs(matrix, i-1,j,dp)+1);
        if(i+1 <=matrix.length-1 && matrix[i+1][j] > cur) //down
            longest = Math.max(longest, dfs(matrix, i+1,j,dp)+1);
        if(j-1>=0 && matrix[i][j-1] > cur) //left
            longest = Math.max(longest, dfs(matrix, i,j-1,dp)+1);
        if(j+1 <=matrix[0].length-1 && matrix[i][j+1] > cur) // right
            longest = Math.max(longest, dfs(matrix, i,j+1,dp)+1);
        dp[i][j] = longest; //set memo
        return dp[i][j];
    }
}