LeetCode: Flatten Binary Tree to Linked List

忘记加tmp->left = NULL了，少数次改

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<TreeNode *> S;
        if (!root) return;
        TreeNode *tmp = root;
        while (tmp) {
            if (tmp->right) S.push(tmp->right);
            if (tmp->left) {
                tmp->right = tmp->left;
                tmp->left = NULL;
                tmp = tmp->right;
            }
            else {
                if (!S.empty()) {
                    tmp->right = S.top();
                    tmp = tmp->right;
                    S.pop();
                }
                else break;
            }
        }
        return;
    }
};

 贴上另外一段

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    void flatten(TreeNode *root) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        stack<TreeNode*> S;
        while (root) {
            if (root->left) {
                if (root->right) S.push(root->right);
                root->right = root->left;
                root->left = NULL;
            }
            else if (!root->right && !S.empty()) {
                root->right = S.top();
                S.pop();
            }
            root = root->right;
        }
    }
};

 C#

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void Flatten(TreeNode root) {
        Stack<TreeNode> S = new Stack<TreeNode>();
        while (root != null) {
            if (root.left != null) {
                if (root.right != null) S.Push(root.right);
                root.right = root.left;
                root.left = null;
            }
            else if (root.right == null && S.Count != 0) {
                root.right = S.Peek();
                S.Pop();
            }
            root = root.right;
        }
    }
}

 Java

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public void flatten(TreeNode root) {
        Stack<TreeNode> st = new Stack<TreeNode>();
        if (root == null) return;
        while (root != null)
        {
            if (root.left != null)
            {
                if (root.right != null) st.push(root.right);
                root.right = root.left;
                root.left = null;
                root = root.right;
            }
            else 
            {
                if (root.right == null && st.empty() == false)
                {
                    root.right = st.peek();
                    st.pop();
                }
                root = root.right;
            }
        }
    }
}