144. 二叉树的前序遍历

已解答

简单

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给你二叉树的根节点 root ，返回它节点值的 前序 遍历。

示例 1：

输入：root = [1,null,2,3]
输出：[1,2,3]

示例 2：

输入：root = []
输出：[]

示例 3：

输入：root = [1]
输出：[1]

示例 4：

输入：root = [1,2]
输出：[1,2]

示例 5：

输入：root = [1,null,2]
输出：[1,2]

提示：

• 树中节点数目在范围 [0, 100] 内
• -100 <= Node.val <= 100

进阶：递归算法很简单，你可以通过迭代算法完成吗？

三个都一样的就改个顺序

 前序

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

前序迭代法 

/*** 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 {// private void perorder(TreeNode root,List<Integer> result){//     if(root == null){//         return;//     }//     result.add(root.val);//     perorder(root.left,result);//     perorder(root.right,result);// }public List<Integer> preorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();if(root == null){return result;}Stack<TreeNode> stack = new Stack<>();stack.push(root);while(!stack.isEmpty()){TreeNode node = stack.pop();result.add(node.val);if(node.right != null){stack.push(node.right);}if(node.left != null){stack.push(node.left);}}return result;}
}

 

中序 

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

中序迭代：

 

/*** 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 {// private void inorder(TreeNode root,List<Integer> result){
//         if(root == null){
//             return;
//         }
//         inorder(root.left,result);
//         result.add(root.val);
//         inorder(root.right,result);//     }public List<Integer> inorderTraversal(TreeNode root) {List<Integer> result = new ArrayList<>();if(root == null){return result;}Stack<TreeNode> stack = new Stack<>();TreeNode cur = root;while(cur != null || !stack.isEmpty()){if(cur != null){stack.push(cur);cur = cur.left;}else{cur = stack.pop();result.add(cur.val);cur = cur.right;}}return result;}
}

后序

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