Introduction
Binary Lifting is a commonly used technique to to efficiently compute powers of a number or perform other exponentiation-related operations. It is particularly useful in algorithms and data structures where you need to repeatedly calculate powers of a value. The binary lifting technique reduces the number of multiplications required to compute the result, making it faster than simple repeated multiplication.
Generic Binary Lifting Template
# M here is the largest power possible
M = k.bit_length() + 1
parent = [[0] * M for _ in range(N)]
for node, p in enumerate(A):
parent[node][0] = p
# construct binary jumping array
for power in range(1, M):
for node in range(N):
parent[node][power] = parent[parent[node][power - 1]][power - 1]Complexity for Generic Binary Lifting
- Time complexity: , where k is the maximum bit length
- Space complexity:
Find Lowest Common Ancestor
M = N.bit_length() + 1
graph = defaultdict(list)
for a, b in edges:
graph[a].append(b)
graph[b].append(a)
parent = [[0] * M for _ in range(N)]
d = [0] * N
def dfs(node, prev, depth):
parent[node][0] = prev
d[node] = depth
for adj in graph[node]:
if adj != prev:
dfs(adj, node, depth + 1)
dfs(0, 0, 0)
# binary lifting
for power in range(1, M):
for node in range(N):
parent[node][power] = parent[parent[node][power - 1]][power - 1]
def lca(a, b):
if d[a] > d[b]:
a, b = b, a
# let a and b jump to the same depth
diff = d[b] - d[a]
for p in range(M):
if diff & (1 << p):
b = parent[b][p]
if a == b: return a
for p in range(M - 1, -1, -1):
if parent[a][p] != parent[b][p]:
a = parent[a][p]
b = parent[b][p]
return parent[a][0]Complexity for Finding LCA
- Time complexity:
- Space complexity: