20. Kth_Snallest_Element.java

/**
 * 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 int kthSmallest(TreeNode root, int k) {
        int ans=inorder(root, k);
        return ans;
    }
    int inorder(TreeNode root, int k) 
    { 
        int count=0;
        if (root == null) 
            return 0; 
  
        Stack<TreeNode> s = new Stack<TreeNode>(); 
        TreeNode curr = root; 
  
        // traverse the tree 
        while (curr != null || s.size() > 0) 
        { 
  
            /* Reach the left most Node of the 
            curr Node */
            while (curr !=  null) 
            { 
                /* place pointer to a tree node on 
                   the stack before traversing 
                  the node's left subtree */
                s.push(curr); 
                curr = curr.left; 
            } 
  
            /* Current must be NULL at this point */
            curr = s.pop();
            count++;
            if(count==k){
                return curr.val;
            }
  
            /* we have visited the node and its 
               left subtree.  Now, it's right 
               subtree's turn */
            curr = curr.right; 
        } 
        return 0;
    }
}