Two Sum IV

Given a Binary Search Tree and a target number, return true if there exist two elements in the BST such that their sum is equal to the given target.

Example 1:

Input: 
    5
   / 
  3   6
 /    
2   4   7

Target = 9

Output: True

Example 2:

Input: 
    5
   / 
  3   6
 /    
2   4   7

Target = 28

Output: False

 解法一： time and space O(n)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean findTarget(TreeNode root, int k) {
        HashSet<Integer> set = new HashSet<Integer>();
        return findTargetHelper(root,set,k);        
    }
    
    public boolean findTargetHelper(TreeNode node, HashSet set, int k)
    {
        if(node == null) return false;
        if(set.contains(k-node.val)) return true;
        set.add(node.val);
        return findTargetHelper(node.left, set, k) || findTargetHelper(node.right, set, k);
        
    }
}

解法二：time and space O(n)。 recursion不需要返回值

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean findTarget(TreeNode root, int k) {
        ArrayList<Integer> list = new ArrayList<Integer>();
        inOrder(root,list);
        int start = 0;
        int end = list.size()-1;
        while(start < end)
        {
            if(list.get(start)+list.get(end) == k) return true;
            if(list.get(start)+list.get(end) > k) end --;
            else start ++;
        }
        
        return false;
    }
    
    public void inOrder(TreeNode root, ArrayList<Integer> list)
    {
        if(root == null) return;
        inOrder(root.left,list);
        list.add(root.val);
        inOrder(root.right,list);
    }
}

解法三：看不懂！

Use the stack and search just like 2sum without dumping all the value out in the beginning.

-- Time/Space: n/logn in average

   public boolean findTarget(TreeNode root, int k) {
        if(root == null) return false;
        Stack<TreeNode> l_stack = new Stack<>();
        Stack<TreeNode> r_stack = new Stack<>();
        stackAdd(l_stack, root, true);
        stackAdd(r_stack, root, false);
        while(l_stack.peek() != r_stack.peek()){
            int n = l_stack.peek().val + r_stack.peek().val;
            if(n == k){
            return true;
            }else if(n > k){
            stackNext(r_stack, false);
            }else{
        stackNext(l_stack, true);
            }
        }
        return false;
    }
    
    private void stackAdd(Stack<TreeNode> stack, TreeNode node, boolean isLeft){
        while(node != null){
            stack.push(node);
            node = (isLeft) ? node.left : node.right;
        }
    }

    private void stackNext(Stack<TreeNode> stack, boolean isLeft){
        TreeNode node = stack.pop();
        if(isLeft){
            stackAdd(stack, node.right, isLeft);
        }else{
            stackAdd(stack, node.left, isLeft);
        }
    }