Day14|第六章 二叉树 part02| 226.翻转二叉树| 101. 对称二叉树| 104.二叉树的最大深度|111.二叉树的最小深度

226.翻转二叉树(递归只能前序或者后序，中序不行)

class Solution {
    public TreeNode invertTree(TreeNode root) {
        if(root == null) return null;
        swap(root);
        invertTree(root.left);
        invertTree(root.right);
        //swap(root);
        return root;
    }
    public void swap(TreeNode root){
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
    }
}

 

01. 对称二叉树：

class Solution {
    public boolean isSymmetric(TreeNode root) {
        return compare(root.left, root.right);
    }
//后序遍历，一定要从下往上对比
    public boolean compare(TreeNode left, TreeNode right){
        if( left == null && right != null){return false;}
        if(left != null && right == null){return false;}
        if(left == null && right == null){return true;}
        if(left.val != right.val){return false;}
        boolean compareOutside = compare(left.left, right.right);
        boolean compareInside = compare(left.right, right.left);
        return compareOutside && compareInside;
    }
}

 

 

104.二叉树的最大深度

class Solution {
    public int maxDepth(TreeNode root) {
        if(root == null) return 0;
        Queue<TreeNode> que = new LinkedList<>();
        int depth = 0;
        que.offer(root);
        while(!que.isEmpty()){
            int size = que.size();
            for(int i = 0; i < size; i++){
                TreeNode temp = que.poll();
                if(temp.left != null){
                    que.offer(temp.left);
                }
                if(temp.right != null){
                    que.offer(temp.right);
                }
            }
            //while 循环一次+1
            depth++;
        }
        return depth;
    }
}

 

111.二叉树的最小深度: 如果一个节点左，右都是空就是最小深度

class Solution {
    public int minDepth(TreeNode root) {
        if(root == null) return 0;
        Queue<TreeNode> que = new LinkedList<>();
        int depth = 0;
        que.offer(root);
        while(!que.isEmpty()){
            //如果左右都为空就返回。
            int size = que.size();
            depth++;
            for(int i = 0; i < size; i++){
                TreeNode temp = que.poll();
                if(temp.left == null && temp.right == null){
                    return depth;
                }
                if(temp.left != null){
                    que.offer(temp.left);
                }
                if(temp.right != null){
                    que.offer(temp.right);
                }
            }
        }
        return depth;
    }
}