第67题 Binary Tree Inorder Traversal

Binary Tree Inorder Traversal

Description
Given a binary tree, return the inorder traversal of its nodes‘ values.

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: Inorder in ArrayList which contains node values.
     */
    public List<Integer> inorderTraversal(TreeNode root) {
        // write your code here
        ArrayList<Integer> results = new ArrayList<Integer>();
        if (root == null){
           return results ;
        }
        List<Integer> left =  inorderTraversal(root.left);
        List<Integer> right =  inorderTraversal(root.right);
        results.addAll(left);
        results.add(root.val);
        results.addAll(right);
        return results ;
    }
}

法2：非递归

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: A Tree
     * @return: Inorder in ArrayList which contains node values.
     */
    public List<Integer> inorderTraversal(TreeNode root) {
        // write your code here
       /* ArrayList<Integer> results = new ArrayList<Integer>();
        if (root == null){
           return results ;
        }
        List<Integer> left =  inorderTraversal(root.left);
        List<Integer> right =  inorderTraversal(root.right);
        results.addAll(left);
        results.add(root.val);
        results.addAll(right);
        return results ;*/
        Stack<TreeNode> stack = new Stack<TreeNode>();
        ArrayList<Integer> results = new ArrayList<Integer>();
        if(root == null){
           return results ;
        }
        TreeNode curt = root ;
        while(curt != null || ! stack.isEmpty()){
           while(curt != null){
           stack.push(curt);
           curt = curt.left ;
           }
           curt = stack.pop();
           results.add(curt.val);
           curt = curt.right;
        }
        return results ;
        
    }
}