代码随想录算法训练营Day11

144.二叉树的前序遍历

递归法

/*** 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> res=new ArrayList<>();preorder(root,res);return res;}public  void preorder(TreeNode root,List<Integer> res){if(root==null){return ;}res.add(root.val);preorder(root.left,res);preorder(root.right,res);}
}

迭代法

/*** 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> result=new ArrayList<>();if(root==null){return result;}  Stack<TreeNode> mystack=new Stack<>();mystack.push(root);while(!mystack.isEmpty()){TreeNode cur=mystack.pop();result.add(cur.val);if(cur.right!=null){mystack.push(cur.right);}if(cur.left!=null){mystack.push(cur.left);}}return result;}
}

94.二叉树的中序遍历

递归法

/*** 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> result=new ArrayList<>();midorder(root,result);return result;}public void midorder(TreeNode root,List<Integer> result){if(root==null){return;}midorder(root.left,result);result.add(root.val);midorder(root.right,result);}
}

迭代法

/*** 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> result=new ArrayList<>();if(root==null){return result;}Stack<TreeNode> mystack=new Stack<>();TreeNode cur=root;while(cur!=null||!mystack.isEmpty()){        if(cur!=null){mystack.push(cur);cur=cur.left;}else{cur=mystack.pop();result.add(cur.val);cur=cur.right;}}return result;}
}

145.二叉树的后序遍历

递归法

/*** 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> result=new ArrayList<>();afterorder(root,result);return result;}public void afterorder(TreeNode root,List<Integer> result){if(root==null){return;}afterorder(root.left,result);afterorder(root.right,result);result.add(root.val);}
}

迭代法

在前序遍历迭代法基础上，改变左右子树节点的入栈顺序，同时翻转输出集合

/*** 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> result=new ArrayList<>();if(root==null){return result;}Stack<TreeNode> mystack=new Stack<>();mystack.push(root);while(!mystack.isEmpty()){TreeNode cur=mystack.pop();result.add(cur.val);if(cur.left!=null){mystack.push(cur.left);}if(cur.right!=null){mystack.push(cur.right);}}Collections.reverse(result);return result;}
}

102.二叉树的层序遍历

/*** 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<List<Integer>> levelOrder(TreeNode root) {List<List<Integer>> result=new ArrayList<List<Integer>>();lo(root,result);return result;}public void lo(TreeNode root,List<List<Integer>> result){if(root==null)return;Deque<TreeNode> myque=new LinkedList<>();myque.offer(root);while(!myque.isEmpty()){List<Integer> curlist=new ArrayList<>();int len=myque.size();while(len>0){TreeNode cur=myque.poll();curlist.add(cur.val);len--;if(cur.left!=null){myque.offer(cur.left);}if(cur.right!=null){myque.offer(cur.right);}} result.add(curlist);      }}
}