---
id: dfs
title: Depth First Search
author: Siyong Huang
prerequisites: 
 - Bronze - Introduction to Graphs
description: A way to traverse a graph using recursion.
---

## Depth First Search

 - [CSES Building Roads](https://cses.fi/problemset/task/1666)

### Tutorial

 - Recommended:
   - CPH 12.1 (DFS), 14 (Tree algorithms)
   - [PAPS 12.2](https://www.csc.kth.se/~jsannemo/slask/main.pdf)
   - [CSAcademy DFS](https://csacademy.com/lesson/depth_first_search/)
 - Additional:
   - [CPC.7](https://github.com/SuprDewd/T-414-AFLV/tree/master/07_graphs_1)
   - [cp-algo DFS](https://cp-algorithms.com/graph/depth-first-search.html)
     - hard to parse if this is your first time learning about DFS
   - [Topcoder Graphs Pt 2](https://www.topcoder.com/community/data-science/data-science-tutorials/introduction-to-graphs-and-their-data-structures-section-2/)

### Problems

 - CSES Trees
   - [Subordinates](https://cses.fi/problemset/task/1674)
     - Tree Traversal
   - [Tree Diameter](https://cses.fi/problemset/task/1131)
   - [Tree Distances I](https://cses.fi/problemset/task/1132)
   - [Tree Distances II](https://cses.fi/problemset/task/1133)
 - CF
   - [PolandBall & Forest](http://codeforces.com/problemset/problem/755/C) [](56)
   - [Bear & Friendship](http://codeforces.com/problemset/problem/771/A)
   - [Journey](http://codeforces.com/contest/839/problem/C) [](54)
     - DFS on Tree
  - [Wizard's Tour](http://codeforces.com/contest/860/problem/D) [](59)
 - USACO
   - [Mootube, Silver (Easy)](http://usaco.org/index.php?page=viewproblem2&cpid=788)
   - [Closing the Barn, Silver (Easy)](http://usaco.org/index.php?page=viewproblem2&cpid=644)
   - [Moocast, Silver (Easy)](http://usaco.org/index.php?page=viewproblem2&cpid=668)
   - [Pails (Normal)](http://usaco.org/index.php?page=viewproblem2&cpid=620)
   - [Milk Visits (Normal)](http://www.usaco.org/index.php?page=viewproblem2&cpid=968)
   - [Count Cross](http://usaco.org/index.php?page=viewproblem2&cpid=716)
   - [Wormhole Sort (Normal)](http://www.usaco.org/index.php?page=viewproblem2&cpid=992)
     - also binary search on the answer
   - [Fence Planning](http://usaco.org/index.php?page=viewproblem2&cpid=944)
   - [Moo Particle](http://www.usaco.org/index.php?page=viewproblem2&cpid=1040)
 - Other
   - [POI Hotels](https://szkopul.edu.pl/problemset/problem/gDw3iFkeVm7ZA3j_16-XR7jI/site/?key=statement) [](61)
   - [Kattis Birthday Party (Easy)](https://open.kattis.com/problems/birthday)
     - DFS with each edge removed
   - [CSA Tree Construction](https://csacademy.com/contest/archive/task/tree-construct)
     - several cases

## Graph Two-Coloring

*Graph two-coloring* refers to assigning a boolean value to each node of the graph, dictated by the edge configuration
The most common example of a two-colored graph is a *bipartite graph*, in which each edge connects two nodes of opposite colors.

 - [CSES Building Teams](https://cses.fi/problemset/task/1668)

### Tutorial

 - CPH 12.3
 
The idea is that we can arbitrarily label a node and then run DFS. Every time we visit a new (unvisited) node, we set its color based on the edge rule. When we visit a previously visited node, check to see whether its color matches the edge rule. For example, an implementation of coloring a bipartite graph is shown below.

```cpp
//UNTESTED

bool is_bipartite = true;
void dfs(int node)
{
  visited[node] = true;
  for(int u:adj_list[node])
    if(visited[u])
    {
      if(color[u] == color[node])
        is_bipartite = false;
    }
    else
    {
      color[u] = !color[node];
      dfs(u);
    }
}
```

 - Additional:
   - [Bipartite Graphs: cp-alg bipartite check](https://cp-algorithms.com/graph/bipartite-check.html)
     - Note: CP-Algorithms uses BFS, but DFS accomplishes the same task

### Problems

 - [CF Bipartiteness](http://codeforces.com/contest/862/problem/B) [](49)
 - [The Great Revegetation (Normal)](http://usaco.org/index.php?page=viewproblem2&cpid=920)