leetcode

Add two Segment Tree implementations to Templates

4 files changed, 85 insertions(+), 10 deletions(-)

diff --git a/ README.md b/ README.md

@@ -6,20 +6,22 @@ In this repo I want to share with you my solutions to some Leetcode problems written in C++ to practice my Algorithms and Data Structure skills. I will

update this list periodically as I do more problems. I hope it will be helpful

update this list periodically as I do more problems. I hope it will be helpful

to some of you if you get stuck on a specific problem. Feel free to contact me if you have any questions. ## Templates

I have accumulated a list of most commonly used algorithms to serve as a base

for solving problems.

I have accumulated a list of most commonly used algorithms to serve as a

reference and a base for solving problems.

* [MST from edge priority queue](Templates/MST_pq.cpp)

* [MST from edge vector](Templates/MST_vector.cpp)

* [Matrix BFS/Flood fill Iterative](Templates/bfs_floodfill.cpp)

* [Matrix BFS/Flood fill Recursive](Templates/bfs_floodfill_recursive.cpp)

* [Union Find](Templates/Union_Find.cpp)

* [MST from edge priority queue](Templates/MST_pq.cpp): Reference implementation

* [MST from edge vector](Templates/MST_vector.cpp): Reference implementation, edges need to be sorted ascending by len

* [Matrix BFS/Flood fill Iterative](Templates/bfs_floodfill.cpp): Reference implementation

* [Matrix BFS/Flood fill Recursive](Templates/bfs_floodfill_recursive.cpp): Reference implementation

* [Union Find](Templates/Union_Find.cpp): Fast implementation, all optimization tricks used

* [Segment Tree Iterative](Templates/segment_tree.cpp): Fast implementation, contains tricky bits

* [Segment Tree Recursive](Templates/segment_tree_recursive.cpp): Reference implementation, easy to understand

## Problems

diff --git a/ Templates/bfs_floodfill_recursive.cpp b/ Templates/bfs_floodfill_recursive.cpp

@@ -8,8 +8,6 @@ int n, m; int valid(int x, int y) { return x >= 0 && x < n && y >= 0 && y < m; } bool dfs(const Matrix &mat, Marked &mark, int a, int b) {

if (got == word.size()) return true;

mark[a][b] = true; for (auto [oa, ob] : offsets) { int x = a + oa, y = b + ob;

diff --git a/ Templates/segment_tree.cpp b/ Templates/segment_tree.cpp

@@ -0,0 +1,25 @@

class SegmentTree {

const int n;

vector<int> t = vector<int>(2 * n);

public:

SegmentTree(const vector<int> nums): n(size(nums)) {

for (int i = 0; i < n; i++) t[n + i] = nums[i];

for (int i = n - 1; i > 0; i--) t[i] = t[i * 2] + t[i * 2 + 1];

}

int sum(int l, int r) const {

int res = 0;

for (l += n, r += n; l < r; l /= 2, r /= 2) {

if (l & 1) res += t[l++];

if (r & 1) res += t[--r];

}

return res;

}

void update(int p, int val) {

for (t[p += n] = val; p > 1; p /= 2) t[p / 2] = t[p] + t[p ^ 1];

}

};

diff --git a/ Templates/segment_tree_recursive.cpp b/ Templates/segment_tree_recursive.cpp

@@ -0,0 +1,50 @@

class SegmentTree {

const int n;

vector<int> t = vector<int>(4 * n);

void build(const vector<int>& nums, int idx, int left, int right) {

if (left == right) {

t[idx] = nums[left];

return;

}

const int mid = (left + right) / 2;

build(nums, idx * 2, left, mid);

build(nums, idx * 2 + 1, mid + 1, right);

t[idx] = t[idx * 2] + t[idx * 2 + 1];

}

int sum(int idx, int left, int right, int l, int r) const {

if (l > r) return 0;

if (l == left && r == right) return t[idx];

const int mid = (left + right) / 2;

return sum(idx * 2, left, mid, l, min(r, mid))

+ sum(idx * 2 + 1, mid + 1, right, max(l, mid + 1), r);

}

void update(int idx, int left, int right, int pos, int val) {

if (left == right) {

t[idx] = val;

return;

}

const int mid = (left + right) / 2;

if (pos <= mid) update(idx * 2, left, mid, pos, val);

else update(idx * 2 + 1, mid + 1, right, pos, val);

t[idx] = t[idx * 2] + t[idx * 2 + 1];

}

public:

SegmentTree(const vector<int> nums): n(size(nums)) {

build(nums, 1, 0, n - 1);

}

int sum(int left, int right) const {

return sum(1, 0, n - 1, left, right);

}

void update(int pos, int val) {

update(1, 0, n - 1, pos, val);

}

};