树的前、中、后、层次遍历-leetcode

 

144. 二叉树的前序遍历

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [1,2,3]

递归：

class Solution {
     List<Integer>lsit = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
       
        if(root == null)
          return  lsit;
            lsit.add(root.val);
            preorderTraversal(root.left);
            preorderTraversal(root.right);
        return lsit;
    }
}

栈

 public List<Integer> preorderTraversal(TreeNode root) {
         if(root == null)
         return new ArrayList<Integer>();
        List<Integer>list = new ArrayList<Integer>();
        Stack<TreeNode>stack = new Stack<TreeNode>();
        stack.push(root);
        while(!stack.empty()){
           TreeNode tnode = stack.pop();
            list.add(tnode.val);
            if(tnode.right != null){
                stack.push(tnode.right);
            }
            if(tnode.left != null){
                stack.push(tnode.left);
            }    
        }
        return list;
    }

94. 二叉树的中序遍历

输入: [1,null,2,3]
   1
    \
     2
    /
   3

输出: [1,3,2]

递归： 

class Solution {
    List<Integer>list = new ArrayList<>();
    public List<Integer> inorderTraversal(TreeNode root) {
        if(root == null)
        return list;
        inorderTraversal(root.left);
        list.add(root.val);
         inorderTraversal(root.right);
          return list;
    }
}

 栈

 public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer>list = new ArrayList<Integer>();
        Stack<TreeNode>stack = new Stack<TreeNode>();
        while(root != null || !stack.empty()){
         while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root.val);
            root = root.right;     
        }
        return list;
    }

 

 145. 二叉树的后序遍历

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 

输出: [3,2,1]

递归： 

class Solution {
    List<Integer>list = new ArrayList<>();
    public List<Integer> postorderTraversal(TreeNode root) {
        if(root == null)
        return list;
        postorderTraversal(root.left);
        postorderTraversal(root.right);
         list.add(root.val);
         return list;
    }
}

栈

  public List<Integer> postorderTraversal(TreeNode root) {
        if(root == null)
            return new ArrayList<Integer>();
        List<Integer>list = new ArrayList<Integer>();
        Stack<TreeNode>stack = new Stack<TreeNode>();
        TreeNode prev = null;
        while(root != null || !stack.empty()){
           while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if (root.right == null || root.right == prev) {
                list.add(root.val);
                prev = root;
                root = null;
            } else {
                stack.push(root);
                root = root.right;
            }

        }
        return list;
    }

102. 二叉树的层序遍历

示例：
二叉树：[3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7
返回其层次遍历结果：

[
  [3],
  [9,20],
  [15,7]
]

广搜（BFS）实现

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) { 
     if(root == null)
        return new ArrayList<>();
      List<List<Integer>> llist= new ArrayList<>();
      Queue<TreeNode>queue = new LinkedList<TreeNode>();
      TreeNode newroot;
      queue.offer(root); 
      while(!queue.isEmpty()){
            List<Integer>list = new ArrayList<>();
            int num =queue.size();
            while(num > 0){
                newroot = queue.poll();
                list.add(newroot.val);
                if(newroot.left != null)
                  queue.offer(newroot.left);
                if(newroot.right != null)
                  queue.offer(newroot.right);
                  num--;
            }
            llist.add(list);
      }
      return llist;
    }
}