深度优先搜索DFS

refer to : https://www.algoexpert.io/questions/Depth-first%20Search

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深度优先搜索

• 分析

深度遍历图结构，找到起始点（根结点），从起始点出发，深度搜索每一个分支，对于每一个分支，一直遍历到该分支的最底层，然后再遍历另外一个分支。

traverse a graph in a DFS way, from the root node, explore a specific branch, the first branch in our graph, all the way down as deep as we can before exploring the next branch。

从根结点出发，首先把根结点加入结果集，然后对根结点的每一个孩子结点调用dfs函数，每到达一个当前结点，把当前结点加入结果集，然后对于当前结点的每一个孩子结点，调用dfs函数。

call the depth-first-search function on each child node , one after the other , it is a sort of recursive way, but not really recursive, we just repeat recall 

when we are at a given node, we will add that node's name into the result array. then for every child in its children nodes, call the dfs function。

left most branch（A, B, E）---> inner left most branch(F, I)--->inner right most branch(J)--->C--->D, G, K--->H

• 代码。  V顶点， E边
• 时间复杂度：
  • O(V + E), 需要遍历每一个结点，V个结点，对于每一个结点，都要一个个遍历它们的孩子结点（for loop to call dfs function），多少条边就有多少个孩子结点。
• 空间复杂度：
  • 结果集是一个v个结点的数组，adding a bunch of frames to call stack, as we go deeper and deeper in a branch, we keep adding frames to the call stack
  • we call dfs function on A, before we resolve this function, we call dfs on B, C, D; when we start with B, we need to call dfs on E and F, then before resolving F, we need to call bfs on I, J. 所以在算出A之前，我们先要算它的所有子结点，最坏的情况是，整个图只有一个大分支，从根部到叶子结点一条大分支，这个时候我们有V个frames on call stack.

• class Node:
      def __init__(self, name):
          self.children = []
          self.name = name
  
      def addChild(self, name):
          self.children.append(Node(name))
          return self
  
      def depthFirstSearch(self, array):
          
          array.append(self.name) //O(V)
          for child in self.children:    //O(E)
              child.depthFirstSearch(array)
          return array

  class Program {
    static class Node {
      String name;
      List<Node> children = new ArrayList<Node>();
  
      public Node(String name) {
        this.name = name;
      }
  
      public List<String> depthFirstSearch(List<String> array) {
        // Write your code here.
              array.add(this.name);
              for(int i = 0; i < children.size();i++){
                  children.get(i).depthFirstSearch(array);
              }
        return array;
      }
  
      public Node addChild(String name) {
        Node child = new Node(name);
        children.add(child);
        return this;
      }
    }
  }