数据结构刷题集合—— 树

Posted 2021-07-24 Johnny*

@TOC

226. 翻转二叉树

【题目描述】
226. 翻转二叉树

【思路】
在遍历过程中，对左右节点进行两两交换。

/**
 * 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 TreeNode invertTree(TreeNode root) {
        
        if( root == null ) return null;
        //交换以left为根的的子树 和以right为根的子树
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        invertTreee(root.left);
        invertTreee(root.right);
        
        return root;
    }
}

中序遍历

/**
 * 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 TreeNode invertTree(TreeNode root) {
        
        if( root == null ) return null;
        invertTree(root.left);
        
        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;
        invertTree(root.left);
        
        return root;
    }
}

【思路】

层次遍历

/**
 * 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 TreeNode invertTree(TreeNode root) {
        
        if( root == null ) return null;
        Queue<TreeNode> q = new LinkedList<>();
        q.offer( root );
        while( !q.isEmpty() ){
            TreeNode node = q.poll();
            TreeNode tmp = node.left;
            node.left = node.right;
            node.right = tmp;
            
            if( node.left != null ) q.offer(node.left);
            if( node.right != null ) q.offer(node.right);
            
        }
        
      
        
        return root;
    }
}