Inorder Preorder Postorder

摘要：题目链接参考这篇文章链接题目链接还是一样的，用，或者保存和。题目链接题目链接按照的顺序最后再一下。

lc题目链接：https://leetcode.com/problems...

recursion：

public class Solution {
    public List inorderTraversal(TreeNode root) {
        List result = new ArrayList();
        dfs(root, result);
        return result;
    }
    
    private void dfs(TreeNode root, List result) {
        // base case
        if(root == null) return;
        dfs(root.left, result);
        result.add(root.val);
        dfs(root.right, result);
    }
}

iteration:

public class Solution {
    public List inorderTraversal(TreeNode root) {
        List result = new ArrayList();
        // stack
        Stack stack = new Stack();
        TreeNode node = root;
        while(!stack.isEmpty() || node != null) {
            while(node != null) {
                stack.push(node);
                node = node.left;
            }
            node = stack.pop();
            result.add(node.val);
            node = node.right;
        }
        return result;
    }
}

morris: 参考这篇文章
http://www.cnblogs.com/AnnieK...

public class Solution {
    public List inorderTraversal(TreeNode root) {
        /* morris: connect the node with its post if post != cur.next(cur.right == null)
         * add current when cur.left == null
         */
         List result = new ArrayList();
         TreeNode prev = null, cur = root;
         while(cur != null) {
             // reach the left most part, just add
             if(cur.left == null) {
                 result.add(cur.val);
                 // right connect to current"s post, null if finish traverse
                 cur = cur.right;
             }
             else {
                 prev = cur.left;
                 while(prev.right != null && prev.right != cur) prev = prev.right;
                 // connect prev with current: connect node with its post
                 if(prev.right == null) {
                     prev.right = cur;
                     cur = cur.left;
                 }
                 // recover the tree
                 else {
                     prev.right = null;
                     result.add(cur.val);
                     cur = cur.right;
                 }
             }
             
         }
         
         return result;
    }
}

链接：https://leetcode.com/problems...

recursion:

public class Solution {
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
        // base case
        if(root == null) return null;
        // left or root
        if(p.val < root.val) {
            TreeNode left = inorderSuccessor(root.left, p);
            return left == null ? root : left;
        }
        else return inorderSuccessor(root.right, p);
    }
}

iteration:

public class Solution {
    public TreeNode inorderSuccessor(TreeNode root, TreeNode p) {
        TreeNode succ = null;
        while(root != null) {
            if(p.val < root.val) {
                succ = root;
                root = root.left;
            }
            else root = root.right;
        }
        
        return succ;
    }
}

题目链接：https://leetcode.com/problems...
还是一样的，iteration用stack，或者morris保存prev和cur。

public class BSTIterator {
    Stack stack;
    public BSTIterator(TreeNode root) {
        stack = new Stack();
        // get the first node
        getAllLeft(root);
    }
    
    private void getAllLeft(TreeNode node) {
        while(node != null) {
            stack.push(node);
            node = node.left;
        }
    }
    
    /** @return whether we have a next smallest number */
    public boolean hasNext() {
        return !stack.isEmpty();
    }

    /** @return the next smallest number */
    public int next() {
        if(!hasNext()) return -1;
        
        TreeNode node = stack.pop();
        if(node.right != null) getAllLeft(node.right);
        return node.val;
    }
}

题目链接：https://leetcode.com/problems...

recursion:

public class Solution {
    public List preorderTraversal(TreeNode root) {
        // recursion
        List result = new ArrayList();
        dfs(root, result);
        return result;
    }
    
    private void dfs(TreeNode root, List result) {
        // base case
        if(root == null) return;
        
        result.add(root.val);
        dfs(root.left, result);
        dfs(root.right, result);
    }
}

iteration:

public class Solution {
    public List preorderTraversal(TreeNode root) {
        // iteration: use stack
        List result = new ArrayList();
        if(root == null)  return result;
        
        Stack stack = new Stack();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            result.add(cur.val);
            
            if(cur.right != null) stack.push(cur.right);
            if(cur.left != null) stack.push(cur.left);
        }
        
        return result;
    }
}

morris:

public class Solution {
    public List preorderTraversal(TreeNode root) {
        // morris: connect cur with its post in inorder
        // add as long as finding the post
        List result = new ArrayList();
        TreeNode cur = root, prev = null;
        while(cur != null) {
            // left is null, need to go to right
            if(cur.left == null) {
                result.add(cur.val);
                cur = cur.right;
            }
            else {
                prev = cur.left;
                // find the post in inorder
                while(prev.right != null && prev.right != cur) prev = prev.right;
                // connect with post and add
                if(prev.right == null) {
                    prev.right = cur;
                    result.add(cur.val);
                    cur = cur.left;
                }
                // recover the tree
                else {
                    prev.right = null;
                    cur = cur.right;
                }
            }
        }
        
        return result;
    }
}

题目链接：https://leetcode.com/problems...

recursion:

public class Solution {
    public List postorderTraversal(TreeNode root) {
        List result = new ArrayList();
        dfs(root, result);
        return result;
    }
    
    private void dfs(TreeNode root, List result) {
        if(root == null)  return;
        
        dfs(root.left, result);
        dfs(root.right, result);
        result.add(root.val);
    }
}

iteration:

public class Solution {
    public List postorderTraversal(TreeNode root) {
        List result = new ArrayList();
        if(root == null)  return result;
        
        // right -> left -> reverse
        Stack stack = new Stack();
        stack.push(root);
        while(!stack.isEmpty()) {
            TreeNode cur = stack.pop();
            result.add(cur.val);
            if(cur.left != null) stack.push(cur.left);
            if(cur.right != null) stack.push(cur.right);
        }
        Collections.reverse(result);
        return result;
    }
}

morris: 按照root -> right -> left的顺序最后再reverse一下。

public class Solution {
    public List postorderTraversal(TreeNode root) {
        List result = new ArrayList();
        // root -> right -> left
        TreeNode cur = root, prev = null;
        while(cur != null) {
            // right == null, go to left
            if(cur.right == null) {
                result.add(cur.val);
                cur = cur.left;
            }
            else {
                prev = cur.right;
                while(prev.left != null && prev.left != cur) {
                    prev = prev.left;
                }
                // connect according to inorder but right first: right->root->left
                // leftmost.left = root
                if(prev.left == null) {
                    result.add(cur.val);
                    prev.left = cur;
                    cur = cur.right;
                }
                else {
                    prev.left = null;
                    cur = cur.left;
                }
            }
        }
        
        Collections.reverse(result);
        return result;
    }
}