代码随想录二刷day14

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前言

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一、力扣二叉树的定义

public class TreeNode{
	private int val;
	private TreeNode left;
	private TreeNode right;
	public void TreeNode(){}
	public void TreeNode(int val){
		this.val = val;	
	}
	public void TreeNode(TreeNode left, TreeNode right, int val){
		this.left = left;
		this.right = right;
		this.val = val;
	}
}

二、力扣144. 二叉树的前序遍历递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        proOrder(list , root);
        return list;
    }
    public void proOrder(List<Integer> list, TreeNode root){
        if(root == null){
            return;
        }
        list.add(root.val);
        proOrder(list, root.left);
        proOrder(list, root.right);
    }
}

三、力扣145. 二叉树的后序遍历递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        postOrder(root, list);
        return list;
    }
    public void postOrder(TreeNode root, List<Integer> list){
        if(root == null){
            return ;
        }
        postOrder(root.left, list);
        postOrder(root.right, list);
        list.add(root.val);
    }
}

四、力扣94. 二叉树的中序遍历递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        inOrder(root, list);
        return list;
    }
    public void inOrder(TreeNode root, List<Integer>list){
        if(root == null){
            return ;
        }
        inOrder(root.left, list);
        list.add(root.val);
        inOrder(root.right, list);
    }
}

五、力扣144. 二叉树的前序遍历迭代

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> preorderTraversal(TreeNode root) {
        Deque<TreeNode> deq = new ArrayDeque<>();
        List<Integer> res = new ArrayList<>();
        TreeNode p = root;
        while(!deq.isEmpty() || p != null){
            if(p != null){
                res.add(p.val);
                deq.offerFirst(p);
                p = p.left;
            }else{
                p = deq.pollFirst();
                p = p.right;
            }
        }
        return res;
    }
}

六、力扣94. 二叉树的中序遍历迭代

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        Deque<TreeNode> deq = new ArrayDeque<>();
        List<Integer> res = new ArrayList<>();
        TreeNode p = root;
        while(!deq.isEmpty() || p != null){
            if(p != null){
                deq.offerFirst(p);
                p = p.left;
            }else{
                p = deq.pollFirst();
                res.add(p.val);
                p = p.right;
            }
        }
        return res;
    }
}

七、力扣145. 二叉树的后序遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> postorderTraversal(TreeNode root) {
        Deque<TreeNode> deq = new ArrayDeque<>();
        List<Integer> res = new ArrayList<>();
        if(root == null)return res;
        deq.offerFirst(root);
        while(!deq.isEmpty()){
            TreeNode p = deq.pollFirst();
            res.add(p.val);
            if(p.left != null)deq.offerFirst(p.left);
            if(p.right != null)deq.offerFirst(p.right);
        }
        Collections.reverse(res);
        return res;
    }
}