This one is from leetcode: link
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]
]
Solution. We can treat each element in matrix as a node. For 2 adjacent nodes, if node1.value < node2.value. Then there is an edge from node1 to node2. In this way, we can transform the matrix to a graph. The problem changes to find the longest path in a directed graph.
In order to find the longest path in graph, each time, we delete the node which out-degree is zero. The number of iteration is the result. Below is a process to calculate the longest path for the graph. The result is 4.
