Pre-order traversal.

Author: Victor

114. Flatten Binary Tree to Linked List.c

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+/*
+114. Flatten Binary Tree to Linked List
+
+Given a binary tree, flatten it to a linked list in-place.
+
+For example, given the following tree:
+
+    1
+   / \
+  2   5
+ / \   \
+3   4   6
+
+
+The flattened tree should look like:
+
+1
+ \
+  2
+   \
+    3
+     \
+      4
+       \
+        5
+         \
+          6
+*/
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     struct TreeNode *left;
+ *     struct TreeNode *right;
+ * };
+ */
+struct TreeNode *preOrderTraversal(struct TreeNode *prev, struct TreeNode *node) {
+    struct TreeNode *left, *right;
+    
+    if (!node) return prev;
+    
+    left = node->left;
+    right = node->right;
+    node->left = node->right = NULL;
+    
+    prev->right = node;
+    prev = node;
+
+    prev = preOrderTraversal(prev, left);   // it's import to update prev
+    prev = preOrderTraversal(prev, right);
+    
+    return prev;
+}
+void flatten(struct TreeNode* root) {
+    struct TreeNode head = { 0 };
+    preOrderTraversal(&head, root);
+}
+
+
+/*
+Difficulty:Medium
+
+
+*/