New Problem Solution - "Smallest String Starting From Leaf"

README.md

@@ -8,6 +8,7 @@ LeetCode
 
 | # | Title | Solution | Difficulty |
 |---| ----- | -------- | ---------- |
+|988|[Smallest String Starting From Leaf](https://leetcode.com/problems/smallest-string-starting-from-leaf/) | [C++](./algorithms/cpp/smallestStringStartingFromLeaf/SmallestStringStartingFromLeaf.cpp)|Medium|
 |985|[Sum of Even Numbers After Queries](https://leetcode.com/problems/sum-of-even-numbers-after-queries/) | [C++](./algorithms/cpp/sumOfEvenNumbersAfterQueries/SumOfEvenNumbersAfterQueries.cpp)|Easy|
 |984|[String Without AAA or BBB](https://leetcode.com/problems/string-without-aaa-or-bbb/) | [C++](./algorithms/cpp/stringWithoutAAAOrBBB/StringWithoutAAAOrBBB.cpp)|Easy|
 |983|[Minimum Cost For Tickets](https://leetcode.com/problems/minimum-cost-for-tickets/) | [C++](./algorithms/cpp/minimumCostForTickets/MinimumCostForTickets.cpp)|Medium|

algorithms/cpp/smallestStringStartingFromLeaf/SmallestStringStartingFromLeaf.cpp

@@ -0,0 +1,69 @@
+// Source : https://leetcode.com/problems/smallest-string-starting-from-leaf/
+// Author : Hao Chen
+// Date   : 2019-02-05
+
+/***************************************************************************************************** 
+ *
+ * Given the root of a binary tree, each node has a value from 0 to 25 representing the letters 'a' to 
+ * 'z': a value of 0 represents 'a', a value of 1 represents 'b', and so on.
+ * 
+ * Find the lexicographically smallest string that starts at a leaf of this tree and ends at the root.
+ * 
+ * (As a reminder, any shorter prefix of a string is lexicographically smaller: for example, "ab" is 
+ * lexicographically smaller than "aba".  A leaf of a node is a node that has no children.)
+ * 
+ * Example 1:
+ * 
+ * Input: [0,1,2,3,4,3,4]
+ * Output: "dba"
+ * 
+ * Example 2:
+ * 
+ * Input: [25,1,3,1,3,0,2]
+ * Output: "adz"
+ * 
+ * Example 3:
+ * 
+ * Input: [2,2,1,null,1,0,null,0]
+ * Output: "abc"
+ * 
+ * Note:
+ * 
+ * 	The number of nodes in the given tree will be between 1 and 1000.
+ * 	Each node in the tree will have a value between 0 and 25.
+ * 
+ ******************************************************************************************************/
+/**
+ * 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 str, result="{"; //'z'+1;
+        smallestFromLeafHelper(root, str, result);
+        return result;
+    }
+    
+    void smallestFromLeafHelper(TreeNode* root, string str, string& result) {
+        if (root->left == NULL && root->right == NULL) {
+            str.insert(0, 1, char(root->val+'a'));
+            result = min(result, str);
+            return;
+        }
+        
+        str.insert(0, 1, char(root->val+'a'));
+        
+        if (root->left) {
+            smallestFromLeafHelper(root->left, str, result);
+        }
+        if (root->right) {
+            smallestFromLeafHelper(root->right, str, result);
+        }
+    }
+};