Efficient Solutions for Counting Good Subsegments in Arrays
Alternative Approach
A key observation is that a subarray [l,r] is valid when max(l,r) - min(l,r) = r - l. This problem appears challenging to maintain efficiently, leading us to consider block decomposition techniques (given the constraints of 1.2e5 elements and 7-second time limit).
Let's divide the array into blocks of size S. We'll handle three cases: left partial blocks (L), right partial blocks (R), and complete middle blocks (M). The solution involves six contribution types: LL, LM, LR, MM, MR, RR, plus special cases where queries fall entirely within a single block.
// Block preprocessing example
void preprocessBlocks() {
for(int block = 1; block <= total_blocks; block++) {
int start = block_size * (block-1) + 1;
int end = min(n, block_size * block);
// Calculate valid subarrays within the block
for(int i = start; i <= end; i++) {
for(int j = i; j <= end; j++) {
if(range_max(i,j) - range_min(i,j) == j - i) {
block_data[block][i-start+1][j-start+1] = 1;
}
}
}
// Compute prefix sums for efficient querying
for(int i = 1; i <= block_size; i++) {
for(int j = 1; j <= block_size; j++) {
block_data[block][i][j] += block_data[block][i-1][j]
+ block_data[block][i][j-1]
- block_data[block][i-1][j-1];
}
}
}
}
The complexity is O(nS + n²/S + qS), where S is optimally set around 1.618√n. This approach requires careful implementation to handle memory constraints and optimize performance.
Optimal Solution
The standard solution uses an offline approach with a sweep line algorithm. We process queries by increasing right andpoints while maintaining answers for all left endpoints.
Key components:
- Maintain max and min values using monotonic stacks
- Track the expression (max-min) - (r-l) wich is always ≥ 0
- Use a segment tree to count zero occurrences efficiently
// Segment tree implementation for counting valid subarrays
struct SegmentTree {
struct Node {
int current_min, min_count;
int historical_min, historical_count;
int lazy_add, historical_lazy;
} tree[N*4];
void push(int node) {
// Propagate lazy updates to children
// Maintain historical minimum information
}
void update(int l, int r, int val, int node=1, int node_l=1, int node_r=n) {
// Apply range updates and maintain statistics
}
int query(int l, int r, int node=1, int node_l=1, int node_r=n) {
// Query historical zero counts in range [l,r]
}
};
The solution achieves O(n log n) complexity by leveraging the segment tree's ability to track historical minimum values and their occurrence counts.