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Author: assassint2017

Smallest String Starting From Leaf/Smallest_String_Starting_From_Leaf.cpp

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+// Runtime: 24 ms, faster than 7.50% of C++ online submissions for Smallest String Starting From Leaf.
+// Memory Usage: 19.6 MB, less than 91.30% of C++ online submissions for Smallest String Starting From Leaf.
+
+/**
+ * Definition for a binary tree node.
+ * struct TreeNode {
+ *     int val;
+ *     TreeNode *left;
+ *     TreeNode *right;
+ *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
+ * };
+ */
+class Solution 
+{
+public:
+    string smallestFromLeaf(TreeNode* root) 
+    {    
+        string curString = "", minString = "";
+        
+        TreeNode* preNode = nullptr;
+        stack<TreeNode*> memo;
+        
+        while (root || !memo.empty())
+        {
+            while (root)
+            {
+                curString += root->val + 'a';
+
+                memo.push(root);
+                root = root->left;
+            }
+            if (!memo.empty())
+            {
+                root = memo.top();
+                memo.pop();
+                
+                if (root->right == nullptr || root->right == preNode)
+                {
+                    if (root->left == nullptr && root->right == nullptr)
+                    {   
+                        if (minString.length() == 0 || smaller(curString, minString))
+                        {
+                            minString = curString;
+                        }
+                    }
+                    
+                    curString.pop_back();
+
+                    preNode = root;
+                    root = nullptr;
+                }
+                else
+                {
+                    memo.push(root);
+                    root = root->right;
+                }
+            }
+        }
+
+        reverse(minString.begin(), minString.end());
+        return minString;
+    }
+private: 
+    bool smaller(const string& str1, const string& str2)
+    {
+        int ptr1 = str1.length() - 1;
+        int ptr2 = str2.length() - 1;
+
+        while (ptr1 >= 0 && ptr2 >= 0)
+        {
+            if (str1[ptr1] < str2[ptr2])
+            {
+                return true;
+            }
+            else if (str1[ptr1] > str2[ptr2])
+            {
+                return false;
+            }
+            else
+            {
+                --ptr1;
+                --ptr2;
+            }
+        }
+
+        return ptr1 >= 0 ? false : true;
+    }
+};