230.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) {
        Deque<TreeNode> stack = new ArrayDeque<TreeNode>();
        while(root!=null||!stack.isEmpty()){
            while(root!=null){
                stack.push(root);
                root=root.left;
            }
            root=stack.pop();
            k--;
            if(k==0)
                return root.val;
            root=root.right;
        }
        return root.val;
    }
}

// 第二次做法，中序遍历以后寻找需要的点
/**
 * 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 {
    private List<Integer> list = new ArrayList<>();
    public int kthSmallest(TreeNode root, int k) {
        if(root==null){
            return 0;
        }
        dfs(root);
        return list.get(k-1);
    }

    private void dfs(TreeNode root){
        if(root==null)
            return;
        dfs(root.left);
        list.add(root.val);
        dfs(root.right);

    }

}