keep going

amazon.cpp

@@ -851,6 +851,17 @@ public:
 
 
 
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+Cut Off Trees for Golf Event
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+//这题凉了，很接近了但是tle解决不了
 class Solution {
 public:
     int cutOffTree(vector<vector<int>>& forest) {
@@ -908,7 +919,7 @@ public:
         pair<int, int> curr = q.front();
         q.pop();
         visited[curr.first][curr.second] = true;
-        if(curr.first!=dest.first || curr.second!=dest.second) total_steps++;
+        if(curr.first==dest.first || curr.second!=dest.second) total_steps++;
         else {return total_steps;}
         //left one
         if(curr.first-1>0 && forest[curr.first-1][curr.second]>1 && !visited[curr.first-1][curr.second]) 
@@ -927,49 +938,6 @@ public:
     }
 };
 
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-//
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 /*#include <iostream>
 #include <vector>
 #include <string>
@@ -1066,4 +1034,105 @@ cout<<cutOffTree(test);
 
 
 	return 0;
-} */
\ No newline at end of file
+} */
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+Letter Combinations of a Phone Number
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+//经典backtracking 问题 （其实就是个recursion问题啦，不难的，考虑好recursion的东西和条件就好了！）
+class Solution {
+public:
+    vector<string> dia = {"0", "1", "abc", "def", "ghi", "jkl", "nmo", "pqrs", "tuv", "wxyz" };
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+    vector<string> letterCombinations(string digits) {
+        vector<string> res;
+        if(digits=="") return res;
+        helper(res, 0, "", digits);
+        return res;
+    }
+    
+    void helper(vector<string> &current, int used_digit, string current_word, string digits){
+        if(used_digit == digits.size()){
+            current.push_back(current_word);
+            return;
+        }
+        else{
+            int current_digit = digits[used_digit]-'0';
+            for(int i=0; i<dia[current_digit].size(); i++){
+                current_word.push_back(dia[current_digit][i]);
+                helper(current, used_digit+1, current_word, digits);
+                // 非常重要！！！backtracking 的精华所在，一定要pop掉最后一个恢复到这个level应有的样子上去！！
+                current_word.pop_back();
+            }
+        }
+    }
+};
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+class Solution {
+public:
+    vector<string> generateParenthesis(int n) {
+        vector<string> res;
+        if(n==0) return res;
+        vector<char> all_parentheses;
+        for(int i=0; i<n; i++){
+            all_parentheses.push_back('(');
+            all_parentheses.push_back(')');
+        }
+        helper(res,"", 0, all_parentheses);
+        return res;
+    }
+    
+    void helper(vector<string>& res, string current_res, int current_used, vector<char>& all_parentheses){
+        if(current_used == all_parentheses.size()) res.push_back(current_res);
+        else{
+            for(int i=0; i<all_parentheses.size(); i++){
+                char add_one = all_parentheses[current_used];
+                current_res.push_back(add_one);
+                helper(res, current_res ,current_used+1, all_parentheses);
+                current_res.pop_back();
+            }
+        }
+    }
+    
+    
+    
+};
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+//
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