Create smallest-string-starting-from-leaf.cpp

Author: kamyu104

C++/smallest-string-starting-from-leaf.cpp

@@ -0,0 +1,36 @@
+// Time:  O(n + l * h), l is the number of leaves
+// Space: O(h)
+
+/**
+ * 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 result = "~";
+        string candidate;
+        dfs(root, &candidate, &result);
+        return result;
+    }
+    
+private:
+    void dfs(TreeNode *node, string *candidate, string *result) {
+        if (!node) {
+            return;
+        }
+
+        candidate->push_back('a' + node->val);
+        if (!node->left && !node->right) {
+            *result = min(*result, string(candidate->rbegin(), candidate->rend()));
+        }
+        dfs(node->left, candidate, result);
+        dfs(node->right, candidate, result);
+        candidate->pop_back();
+    }
+};