add flashcard generation tooling and binary search cards

- gen-flashcards.py: auto-generate recognition cards from all problem files
- toolkit/gen-problem-cards.org: 199 auto-generated problem cards
- 5 binary search tool cards (std::binary_search, std::lower_bound, comparison, two-sum pattern, sorting gotcha)
- two-sum.org: add binary search C++ attempt
- lc-org.el: add doom emacs localleader keybinding support

org/study_deck_02/dsa/arrays-hashing/0001-two-sum.org

@@ -67,10 +67,27 @@ class Solution:
 
 ** TODO C++
 #+begin_src cpp :lc-problem 1
+#include <algorithm>
 class Solution {
 public:
-    vector<int> twoSum(vector<int>& nums, int target) {
-        
+  vector<int> twoSum(vector<int>& nums, int target) {
+    std::sort(nums.begin(), nums.end());
+    for (int i=0; i< nums.size(); i++) {
+      int x = nums[i];
+      if (x > target) {
+        return {};
+      }
+      // TODO: c++ binary search, i forgot how to do this
+      int want = target - x
+      auto it = std::binary_learch(nums.begin() + i + 1, nums.end(), want);
+      if (it != nums.end() && *it == want) {
+        int wi = std::distance(nums.begin(), it)
+        return {i, wi}
+      }
     }
+    reuturn {};
+  }
 };
 #+end_src
+
+#+RESULTS:

org/study_deck_02/dsa/binary-search/lower-bound-vs-binary-search.org

@@ -0,0 +1,11 @@
+#+ANKI_DECK: study_deck_02
+* When to use lower_bound vs binary_search :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+When should you use ~std::lower_bound~ instead of ~std::binary_search~?
+
+** Back
+Use ~lower_bound~ when you need the *position/index* of the element.
+Use ~binary_search~ when you only need to know *if it exists* (bool).

org/study_deck_02/dsa/binary-search/sorting-destroys-indices.org

@@ -0,0 +1,10 @@
+#+ANKI_DECK: study_deck_02
+* Two Sum gotcha: sorting destroys original indices :cpp:two-sum:binary-search:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+If you sort ~nums~ in-place for Two Sum, what goes wrong?
+
+** Back
+Sorting changes the indices. If the problem requires returning *original* indices, store ~{value, original_index}~ pairs before sorting.

org/study_deck_02/dsa/binary-search/std-binary-search.org

@@ -0,0 +1,14 @@
+#+ANKI_DECK: study_deck_02
+* std::binary_search: check element exists :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+What does ~std::binary_search~ return, and what must the range be?
+
+** Back
+Returns ~bool~ (true if element found). The range must be *sorted*.
+
+#+begin_src cpp
+bool found = std::binary_search(vec.begin(), vec.end(), value);
+#+end_src

org/study_deck_02/dsa/binary-search/std-lower-bound.org

@@ -0,0 +1,14 @@
+#+ANKI_DECK: study_deck_02
+* std::lower_bound: find position of element :cpp:binary-search:algorithm:retrieval::recognition:
+:PROPERTIES:
+:END:
+
+** Front
+What does ~std::lower_bound~ return?
+
+** Back
+An iterator to the first element *>=* the given value. Returns ~end()~ if no such element exists.
+
+#+begin_src cpp
+auto it = std::lower_bound(vec.begin(), vec.end(), value);
+#+end_src

org/study_deck_02/dsa/binary-search/two-sum-lower-bound.org

@@ -0,0 +1,16 @@
+#+ANKI_DECK: study_deck_02
+* Task: Two Sum complement lookup with lower_bound :cpp:binary-search:two-sum:retrieval::production:
+:PROPERTIES:
+:END:
+
+** Front
+Given sorted ~nums~ and index ~i~, write C++ to check if ~target - nums[i]~ exists in the rest of the array using ~std::lower_bound~.
+
+** Back
+#+begin_src cpp
+int complement = target - nums[i];
+auto it = std::lower_bound(nums.begin() + i + 1, nums.end(), complement);
+if (it != nums.end() && *it == complement) {
+    return {(int)i, (int)std::distance(nums.begin(), it)};
+}
+#+end_src