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226.翻转二叉树

力扣题目链接：. - 力扣（LeetCode）

DFS（前序递归）

在前序遍历的基础上，实现交换即可

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

BFS

层序遍历

class Solution {public TreeNode invertTree(TreeNode root) {if(root==null){return null;}Deque<TreeNode> myqueue=new LinkedList<>();myqueue.offer(root);while(!myqueue.isEmpty()){int len=myqueue.size();while(len>0){TreeNode cur=myqueue.poll();swap(cur);if(cur.left!=null){myqueue.push(cur.left);}if(cur.right!=null){myqueue.push(cur.right);}len--;}}return root;}public void swap(TreeNode tn){TreeNode temp=tn.left;tn.left=tn.right;tn.right=temp;}
}

101. 对称二叉树

力扣题目链接：. - 力扣（LeetCode）

DFS(后序递归）

class Solution {public boolean isSymmetric(TreeNode root) {return ifsame(root.left,root.right);}public boolean ifsame(TreeNode left,TreeNode right){if(left==null&&right!=null){return false;}else if(right==null&&left!=null){return false;}else if(right==null&&left==null){return true;}else if(right.val!=left.val){return false;}else{boolean out=ifsame(left.left,right.right);boolean in=ifsame(left.right,right.left);return out&∈}}
}

104.二叉树的最大深度

BFS

class Solution {public int maxDepth(TreeNode root) {if(root==null){return 0;}Deque<TreeNode> myqueue=new LinkedList<>();myqueue.offer(root);int depth=0;while(!myqueue.isEmpty()){depth++;int len=myqueue.size();while(len>0){TreeNode cur=myqueue.poll();if(cur.right!=null){myqueue.offer(cur.right);}if(cur.left!=null){myqueue.offer(cur.left);}len--;}}return depth;}
}

111.二叉树的最小深度

BFS

class Solution {public int minDepth(TreeNode root) {if(root==null){return 0;}Deque<TreeNode> myqueue=new LinkedList<>();myqueue.offer(root);int minDepth=0;while(!myqueue.isEmpty()){int len=myqueue.size();minDepth++;while(len>0){TreeNode cur=myqueue.poll();boolean existleft=false;boolean existright=false;if(cur.left!=null){myqueue.offer(cur.left);existleft=true;}if(cur.right!=null){myqueue.offer(cur.right);existright=true;}if(!(existleft||existright))return minDepth;len--;}}return minDepth;}
}