I am trying to understand the concept of lazy propagation in segment trees but I am unable to
grasp the core concept behind it please help me out!!!
(ps. especially the part where the node is updated
by multiplying (end-start+1)*val )
void updateRange(int node, int start, int end, int l, int r, int val)
{
if(lazy[node] != 0)
{
// This node needs to be updated
tree[node] += (end - start + 1) * lazy[node]; // Update it
if(start != end)
{
lazy[node*2] += lazy[node]; // Mark child as lazy
lazy[node*2+1] += lazy[node]; // Mark child as lazy
}
lazy[node] = 0; // Reset it
}
if(start > end or start > r or end < l) // Current segment is not within range [l, r]
return;
if(start >= l and end <= r)
{
// Segment is fully within range
tree[node] += (end - start + 1) * val;
if(start != end)
{
// Not leaf node
lazy[node*2] += val;
lazy[node*2+1] += val;
}
return;
}
int mid = (start + end) / 2;
updateRange(node*2, start, mid, l, r, val); // Updating left child
updateRange(node*2 + 1, mid + 1, end, l, r, val); // Updating right child
tree[node] = tree[node*2] + tree[node*2+1]; // Updating root with max value
}
int queryRange(int node, int start, int end, int l, int r)
{
if(start > end or start > r or end < l)
return 0; // Out of range
if(lazy[node] != 0)
{
// This node needs to be updated
tree[node] += (end - start + 1) * lazy[node]; // Update it
if(start != end)
{
lazy[node*2] += lazy[node]; // Mark child as lazy
lazy[node*2+1] += lazy[node]; // Mark child as lazy
}
lazy[node] = 0; // Reset it
}
if(start >= l and end <= r) // Current segment is totally within range [l, r]
return tree[node];
int mid = (start + end) / 2;
int p1 = queryRange(node*2, start, mid, l, r); // Query left child
int p2 = queryRange(node*2 + 1, mid + 1, end, l, r); // Query right child
return (p1 + p2);
}
. Thanks in advance!