Saturday, February 7, 2015

LeetCode [144] Binary Tree Preorder Traversal

 144. Binary Tree Preorder Traversal

Medium

Given the root of a binary tree, return the preorder traversal of its nodes' values.

 

Example 1:

Input: root = [1,null,2,3]
Output: [1,2,3]

Example 2:

Input: root = []
Output: []

Example 3:

Input: root = [1]
Output: [1]

Example 4:

Input: root = [1,2]
Output: [1,2]

Example 5:

Input: root = [1,null,2]
Output: [1,2]

 

Constraints:

  • The number of nodes in the tree is in the range [0, 100].
  • -100 <= Node.val <= 100

 

Follow up: Recursive solution is trivial, could you do it iteratively?


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/**
 * 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> list = new ArrayList<>();
        if(root==null) return list;
        TreeNode p = root;
        
        while(p!=null){
            if(p.left==null){
                list.add(p.val);
                p = p.right;
            }else{//p.left!=null;
                TreeNode prev = p.left;
                while(prev.right!=null && prev.right!=p) prev = prev.right;
                if(prev.right==null){
                    prev.right=p;
                    list.add(p.val);
                    p = p.left;
                }else{
                    prev.right=null;
                    p = p.right;
                }
            }
        }
        
        return list;
    }
}


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/**
 * 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 {
    List<Integer> list = new ArrayList<>();
    public List<Integer> preorderTraversal(TreeNode root) {
        helper(root);
        return list;
    }
    
    void helper(TreeNode node){
        if(node==null) return;
        list.add(node.val);
        helper(node.left);
        helper(node.right);
        
    }
}


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/**
 * 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> list = new ArrayList<>();
        if(root==null) return list;
        
        TreeNode H = new TreeNode(0);
        H.left = root;
        TreeNode p = H, pred, n1, n2, n3;
        while(p!=null){
            if(p.left==null){
                p = p.right;
            }else{//p.left!=null
                pred = p.left;
                while(pred.right!=null && pred.right!=p) pred = pred.right;
                if(pred.right==null){
                    pred.right = p;
                    p = p.left;
                }else{
                    n1 = p;
                    n2 = n1.left;
                    while(n2!=p){
                        n3 = n2.right;
                        n2.right = n1;
                        n1 = n2;
                        n2 = n3;
                    }
                    
                    n1 = p;
                    n2 = pred;
                    while(n2!=p){
                        list.add(n2.val);
                        n3 = n2.right;
                        n2.right = n1;
                        n1 = n2;
                        n2 = n3;
                    }
                    
                    pred.right = null;
                    p = p.right;
                }
            }
        }
        return list;
    }
}
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