leetcode - 二叉树的遍历（前序、中序、后序、层序）（递归、迭代）

• 二叉树前序遍历（递归 + 迭代）

  vector<int> res;
  vector<int> preorderTraversal(TreeNode* root){
      preorder(root);
      return res;
  }
  
  void preorder(TreeNode* root){
      if(!root) return;
      res.push_back(root -> val);
      preorder(root -> left);
      preorder(root -> right);
  }

  vector<int> preorderTraversal(TreeNode* root){
      vector<int> res;
      stack<TreeNode*> stk;
      TreeNode* cur = root;
      while(cur || !stk.empty()){
          while(cur){
              stk.push(cur);
              res.push_back(cur -> val);
              cur = cur -> left;
          }
          cur = stk.top();
          stk.pop();
          cur = cur -> right;
      }
      return res;
  }

• 二叉树中序遍历（递归 + 迭代）

  vector<int> res;
  vector<int> inorderTraversal(TreeNode* root){
      inorder(root);
      return res;
  }
  
  void inorder(TreeNode* root){
      if(!root) return;
      inorder(root -> left)
      res.push_back(root -> val);
      inorder(root -> right);
  }

  vector<int> inorderTraversal(TreeNode* root){
      vector<int> res;
      stack<TreeNode*> stk;
      TreeNode* cur = root;
      while(cur || !stk.empty()){
          while(cur){
              stk.push(cur);
              cur = cur -> left;
          }
          cur = stk.top();
          stk.pop();
          res.push_back(cur -> val);
          cur = cur -> right;
      }
      return res;
  }

• 二叉树后序遍历（递归 + 迭代）

  vector<int> res;
  vector<int> postorderTraversal(TreeNode* root){
      postorder(root);
      return res;
  }
  
  void postorder(TreeNode* root){
      if(!root) return;
      preorder(root -> left);
      preorder(root -> right);
      res.push_back(root -> val);
  }

  vector<int> postorderTraversal(TreeNode* root){
      vector<int> res;
      stack<TreeNode*> stk;
      TreeNode* cur = root;
      while(cur || !stk.empty()){
          while(cur){
              stk.push(cur);
              res.push_back(cur -> val);
              cur = cur -> right;
          }
          cur = stk.top();
          stk.pop();
          cur = cur -> left;
      }
      reverse(res.begin(), res.end());
      return res;
  }

• 二叉树层序遍历（BFS + DFS）

  vector<vector<int>> levelOrder(TreeNode* root) {
      queue<TreeNode*> q;
      vector<vector<int>> res;
      q.push(root);
      while(!q.empty())
      {
          int n = q.size();
          vector<int> level;
          for(int i = 0; i < n; ++ i)
          {
              TreeNode* cur = q.front();
              q.pop();
              level.push_back(cur -> val);
              if(cur -> left) q.push(cur -> left);
              if(cur -> right) q.push(cur -> right);
          }
          res.push_back(level);
      }
      return res;
  }

  vector<vector<int>> res;
  vector<vector<int>> levelOrder(TreeNode* root) {
      levelorder(root, 0);
      return res;
  }
  
  void levelorder(TreeNode* root, int level){
      if(!root) return;
      if(level == res.size()) res.emplace_back();
      res[level].push_back(root -> val);
      levelorder(root -> left, level + 1);
      levelorder(root -> right, level + 1);
  }