二叉树的遍历

1.中序遍历

https://leetcode-cn.com/problems/binary-tree-inorder-traversal/submissions/

1.1递归

微信图片_20191017204657_wps图片.jpg

class Solution {
    List result;
    public List inorderTraversal(TreeNode root) {
        result =new ArrayList<>();
        if(root==null) return result;
        getMyFixGroup(root);
        return result;
    }

    private void getMyFixGroup(TreeNode root) {
       //加入左边
        if (root.left != null) {
            getMyFixGroup( root.left );
        }
      //加入根
        result.add( root.val );
     //加入右边
        if (root.right != null) {
            getMyFixGroup( root.right );
        }
    }
}

1.2非递归

微信图片_20191017204709.jpg

class Solution {
   public List inorderTraversal(TreeNode root) {
        List res = new ArrayList<>();
        Stack wait = new Stack<>();
        boolean isRight = false;
        while(!wait.empty()||root!=null){  //1.当当前节点或者栈内的节点不为空的时候,循环继续
           if(root!=null){    //2.节点不为null
               if(!isRight&&root.left!=null){//3节点的左子树不为null且不是栈内取出的节点
                   wait.add(root);  //4.根节点加入栈
                   root = root.left; //5.当前节点置为根的左节点
               }else{
                   res.add(root.val);  //6.左节点为null,则根加入结果,并置当前节点为根的右节点
                   root = root.right;
               }
               isRight = false;  //7.不是从栈内取出的标志位
           }else{
               root = wait.pop();  //8.当前的节点为空,但栈不为空,从栈内弹出一个节点
               isRight = true; //9,栈内取出的标志位为true
           }
        }
        return res;

    }
}

剩下的遍历和中序遍历类似

2.前序遍历

https://leetcode-cn.com/problems/binary-tree-preorder-traversal/

2.1递归

class Solution {
     List result;
    public List preorderTraversal(TreeNode root) {
        result = new ArrayList<>();
        checkMyNode(root);
        return result;
    }

    private void checkMyNode(TreeNode root) {
        if(root == null)return;
        result.add(root.val);
        checkMyNode(root.left);
        checkMyNode(root.right);
    }
}

2.2非递归

class Solution {
    public List preorderTraversal(TreeNode root) {
        Stack wait = new Stack<>();
        List res = new ArrayList<>();
 //1.当当前节点或者栈内的节点不为空的时候,循环继续
        while(root!=null||!wait.isEmpty()){
            if(root!=null){
//2.根节点加入结果
                res.add(root.val);
//3节点的右子树不为null
                if(root.right!=null){
//4.加入待处理节点
                    wait.add(root.right);
                }
//5.根节点更新为根的左节点
                root = root.left;
            }else{
//6.如果当前的节点为null,从栈节点返回
                root = wait.pop();
            }
        }
        return res;
    }
}

3.后序遍历

https://leetcode-cn.com/problems/binary-tree-postorder-traversal/

3.1递归

class Solution {
    List result;
    public List postorderTraversal(TreeNode root) {
        result = new ArrayList<>();
        if(root != null) {
            checkMyNode(root);
        }
        return result;
    }

    private void checkMyNode(TreeNode root) {
        if(root == null)return;
        checkMyNode(root.left);
        checkMyNode(root.right);
        result.add(root.val);
    }
}

3.2非递归

class Solution {
   public List postorderTraversal(TreeNode root) {
//1.待处理的节点
        Stack wait = new Stack<>();
//2.该节点左右是否已经处理的标志
        Stack flag = new Stack<>();
//3.结果
        List res = new ArrayList<>();
        boolean isDeal = false;
        while(root!=null||!wait.isEmpty()){
            if(root!=null){
                if(!isDeal){
                  //1.当前节点未处理,且左右节点的各种情况
                    if(root.left == null&&root.right == null){
                        res.add(root.val);
                        root = null;
                    }else if(root.left==null&&root.right!=null){
                        wait.add(root);
                        flag.add(true);
                        root = root.right;
                    }else if(root.left!=null&&root.right==null){
                        wait.add(root);
                        flag.add(true);
                        root = root.left;
                    }else if(root.left!=null&&root.right!=null){
                        wait.add(root);
                        flag.add(true);
                        wait.add(root.right);
                        flag.add(false);
                        root = root.left;
                    }
                    isDeal = false;
                }else{
//当前节点的左右节点都已经处理过了,直接接入结果
                    res.add(root.val);
                    if(!wait.empty()){
                        root = wait.pop();
                        isDeal = flag.pop();
                    }else{
                        root = null;
                    }
                }
            }else{
//从待处理的栈中弹出结果
                root = wait.pop();
                isDeal = flag.pop();
            }
        }
        return res;

    }
}