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

org/study_deck_02/gen-flashcards.py

@@ -0,0 +1,158 @@
+#!/usr/bin/env python3
+"""Auto-generate Anki flashcards from problem .org files.
+
+Generates one recognition card per problem:
+  Front: Problem number + name
+  Back:  Problem statement (first paragraph)
+"""
+
+import re
+import sys
+from pathlib import Path
+
+BASE = Path(__file__).parent
+DSA = BASE / "dsa"
+OUTPUT = BASE / "toolkit" / "gen-problem-cards.org"
+
+TOPIC_DISPLAY = {
+    "1-d-dynamic-programming": "1D DP",
+    "2-d-dynamic-programming": "2D DP",
+    "advanced-graphs": "Advanced Graphs",
+    "arrays-hashing": "Arrays & Hashing",
+    "backtracking": "Backtracking",
+    "binary-search": "Binary Search",
+    "bit-manipulation": "Bit Manipulation",
+    "graphs": "Graphs",
+    "greedy": "Greedy",
+    "heap-priority-queue": "Heap / Priority Queue",
+    "intervals": "Intervals",
+    "linked-list": "Linked List",
+    "math-geometry": "Math & Geometry",
+    "sliding-window": "Sliding Window",
+    "stack": "Stack",
+    "trees": "Trees",
+    "tries": "Tries",
+    "two-pointers": "Two Pointers",
+}
+
+
+def parse_file(path: Path) -> dict | None:
+    """Extract problem metadata and statement from an .org file."""
+    text = path.read_text()
+
+    # Match heading: * TODO 0217. Contains Duplicate :easy:
+    m = re.search(
+        r"^\* (?:TODO|DONE) (\d{4})\. (.+?) :(easy|medium|hard):",
+        text,
+        re.MULTILINE,
+    )
+    if not m:
+        return None
+
+    num = m.group(1)
+    name = m.group(2).strip()
+    difficulty = m.group(3)
+
+    # Topic = parent directory name
+    topic_slug = path.parent.name
+    topic = TOPIC_DISPLAY.get(topic_slug, topic_slug.replace("-", " ").title())
+
+    # Problem statement: first non-empty, non-heading, non-property line after :END:
+    after_props = text[m.end():]
+    lines = after_props.split("\n")
+    statement_lines = []
+    in_block = False
+    for line in lines:
+        stripped = line.strip()
+        if stripped == ":END:":
+            continue
+        if not stripped:
+            if statement_lines:
+                break
+            continue
+        if stripped.startswith("#+") or stripped.startswith("*") or stripped.startswith(":"):
+            if statement_lines:
+                break
+            continue
+        statement_lines.append(stripped)
+
+    statement = " ".join(statement_lines).strip()
+    # Clean up org markup for plain text
+    statement = re.sub(r"[~=*/_]", "", statement)
+
+    return {
+        "num": num,
+        "name": name,
+        "difficulty": difficulty,
+        "topic": topic,
+        "statement": statement,
+        "path": path,
+    }
+
+
+def generate_cards(problems: list[dict]) -> str:
+    """Generate org-mode flashcard content."""
+    parts = []
+    parts.append("#+TITLE: Auto-Generated Problem Cards")
+    parts.append("#+ANKI_DECK: study_deck_02")
+    parts.append("")
+
+    for p in sorted(problems, key=lambda x: int(x["num"])):
+        num = p["num"]
+        name = p["name"]
+        diff = p["difficulty"]
+        topic = p["topic"]
+        stmt = p["statement"]
+
+        # Card: given problem name → what does it ask?
+        title = f"LC {num}. {name} — what does it ask?"
+        tags = f":leetcode:{diff}:{topic.replace(' ', '-').lower()}:retrieval::recognition:"
+
+        parts.append(f"* {title} {tags}")
+        parts.append(":PROPERTIES:")
+        parts.append(":ANKI_NOTE_TYPE: Basic")
+        parts.append(":END:")
+        parts.append("** Front")
+        parts.append(f"What does LeetCode {num} *{name}* ask you to do?")
+        parts.append("** Back")
+        parts.append(stmt)
+        parts.append("")
+
+    return "\n".join(parts)
+
+
+def main():
+    dry_run = "--dry-run" in sys.argv
+
+    org_files = sorted(DSA.rglob("*.org"))
+    org_files = [f for f in org_files if f.name != "udfs.org"]
+    print(f"Found {len(org_files)} problem files")
+
+    problems = []
+    skipped = 0
+    for f in org_files:
+        p = parse_file(f)
+        if p:
+            problems.append(p)
+        else:
+            skipped += 1
+            print(f"  skipped: {f.name}")
+
+    print(f"Parsed: {len(problems)} problems, {skipped} skipped")
+
+    content = generate_cards(problems)
+
+    if dry_run:
+        print(f"\nWould write {len(problems)} cards to {OUTPUT}")
+        # Show first 3 cards as preview
+        preview = content.split("\n\n\n")[:3]
+        print("\n--- Preview ---")
+        print("\n\n".join(preview))
+    else:
+        OUTPUT.parent.mkdir(parents=True, exist_ok=True)
+        OUTPUT.write_text(content)
+        print(f"\nWrote {len(problems)} cards to {OUTPUT}")
+
+
+if __name__ == "__main__":
+    main()

org/study_deck_02/lc-org.el

@@ -71,22 +71,32 @@ Returns a plist (:problem :lang :input :code) or signals an error."
                    (display-buffer (process-buffer proc)))))
     (display-buffer buf)))
 
-(defvar lc-org-mode-map
-  (let ((map (make-sparse-keymap))
-        (prefix (make-sparse-keymap)))
+(defvar lc-org-mode-map (make-sparse-keymap)
+  "Keymap for `lc-org-mode'.")
+
+(when (and (boundp 'doom-version) (fboundp 'map!))
+  (map! :map lc-org-mode-map
+        :localleader
+        :prefix "l"
+        "s" #'lc-org-submit
+        "r" #'lc-org-run
+        "t" #'lc-org-status
+        "p" #'lc-org-show-problem
+        "d" #'lc-org-daily))
+
+(unless (and (boundp 'doom-version) (fboundp 'map!))
+  (let ((prefix (make-sparse-keymap)))
     (define-key prefix (kbd "s") #'lc-org-submit)
     (define-key prefix (kbd "r") #'lc-org-run)
     (define-key prefix (kbd "t") #'lc-org-status)
     (define-key prefix (kbd "p") #'lc-org-show-problem)
     (define-key prefix (kbd "d") #'lc-org-daily)
-    (define-key map (kbd "C-c l") prefix)
-    map)
-  "Keymap for `lc-org-mode'.")
+    (define-key lc-org-mode-map (kbd "C-c l") prefix)))
 
 ;;;###autoload
 (define-minor-mode lc-org-mode
   "Minor mode for LeetCode CLI integration in org source blocks.
-Keybindings under C-c l:
+Keybindings under localleader l (SPC m l or , l):
   s  submit   — submit current block
   r  run      — run with test input
   t  status   — check submission status