add it to inc exc now

library/data_structures_[l,r)/seg_tree.hpp

@@ -5,6 +5,9 @@
 //!   tree st2(n, INT_MAX, [&](int x, int y) -> int {
 //!     return min(x, y);
 //!   });
+//!   int idx = st2.walk(l, r, [&](int value) {
+//!     return value > x;
+//!   }); // min idx in [l, r) such that f is false
 //! @endcode
 //! @time O(n + q log n)
 //! @space O(n)
@@ -27,4 +30,5 @@ template<class T, class F> struct tree {
     }
     return op(x, y);
   }
+#include "seg_tree_uncommon/walk.hpp"
 };

library/data_structures_[l,r)/seg_tree_uncommon/walk.hpp

@@ -0,0 +1,8 @@
+int walk(int l, int r, const auto& f) {
+  while (l < r) {
+    int u = l + n, x = __lg(min(u & -u, r - l));
+    if (f(s[u >> x])) l += 1 << x;
+    else r = l + (1 << x) - 1;
+  }
+  return l;
+}

tests/library_checker_aizu_tests/data_structures/simple_tree_walk.test.cpp

@@ -0,0 +1,44 @@
+#define PROBLEM \
+  "https://judge.yosupo.jp/problem/predecessor_problem"
+#include "../template.hpp"
+#include "../../../library/data_structures_[l,r)/seg_tree.hpp"
+int main() {
+  cin.tie(0)->sync_with_stdio(0);
+  int n, q;
+  cin >> n >> q;
+  tree st(n, 0, plus<int>{});
+  string s;
+  cin >> s;
+  for (int i = 0; i < n; i++) st.update(i, s[i] - '0');
+  while (q--) {
+    int type, k;
+    cin >> type >> k;
+    if (type == 0) {
+      if (st.query(k, k + 1) == 0) st.update(k, 1);
+    } else if (type == 1) {
+      if (st.query(k, k + 1) == 1) st.update(k, 0);
+    } else if (type == 2) {
+      cout << st.query(k, k + 1) << '\n';
+    } else if (type == 3) {
+      // returns first element in [k,n) such that sum > 0
+      int idx =
+        st.walk(k, n, [&](int sum) { return sum == 0; });
+      if (idx == n) idx = -1;
+      cout << idx << '\n';
+    } else {
+      assert(type == 4);
+      int total = st.query(0, k + 1);
+      if (total == 0) {
+        cout << -1 << '\n';
+      } else {
+        int pref_sum = 0;
+        cout << st.walk(0, k + 1, [&](int sum) {
+          if (pref_sum + sum < total)
+            return pref_sum += sum, 1;
+          return 0;
+        }) << '\n';
+      }
+    }
+  }
+  return 0;
+}