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算法 | 分治 - Divide and Conquer

. 5 min read

Divide and Conquer



Break up problem into several parts.

Solve each part recursively.

Combine solutions to sub-problems into overall solution.

Most common usage.

  • Break up problem of size n into two equal parts of size ½ n.
  • Solve two parts recursively.
  • Combine two solutions into overall solution in linear time.


  • Brute force: $n^2$.
  • Divide-and-conquer: $n log n$.


  • Divide array into two halves.
  • Recursively sort each half.
  • Merge two halves to make sorted whole.

Challenge for the bored. In-place merge. [Kronrud, 1969]

using only a constant amount of extra storage

Def. T(n) = number of comparisons to mergesort an input of size n.

Mergesort recurrence:
\text{0,}&\quad\text{if n = 1,}\
T(\lceil \frac{n}{2}) + T(\lfloor \frac{n}{2} \rfloor) + n, &\quad\text{otherwise.}
$T(n) = O(n log_2n). $

Counting Inversions



Divide: separate list into two pieces.

Conquer: recursively count inversions in each half.

Combine: count inversions where $a_i$ and $a_j$ are in different halves, and return sum of three quantities

&Sort-and-Count(L)\quad{ \
&\qquad\text{if list L has one element}\
&\qquad\qquad \text{return 0 and the list L}\
&\qquad \text{Divide the list into two halves A and B}\
&\qquad (r_A, A) \leftarrow Sort-and-Count(A)\
&\qquad (r_B, B) \leftarrow Sort-and-Count(B)\
&\qquad (r_B, L) \leftarrow Merge-and-Count(A, B)\

Closest Pair of Points


Closest pair: Given n points in the plane, find a pair with smallest Euclidean distance between them


  • Divide: draw vertical line L so that roughly ½ n points on each side.
  • Conquer: find closest pair in each side recursively
  • Combine: find closest pair with one point in each side.【利用鸽巢原理剪枝,减少需要判断的数量】
  • Return best of 3 solutions.

优化 combine 步骤:

Find closest pair with one point in each side, assuming that $distance < \delta = min{左右两个部分的最近点}$.

  • So, only need to consider points within $\delta$ of line L


Running Time
T(n) &\leq 2T(\frac{n}{2}) + O(nlogn)\
More optimal:

  • Don't sort points in strip from scratch each time.
  • Each recursive returns two lists: all points sorted by y coordinate,and all points sorted by x coordinate.
  • Sort by merging two pre-sorted lists.

T(n) &\leq 2T(\frac{n}{2}) + O(n)\


Convolution and FFT

FFT: Fast way to convert between time-domain and frequency-domain.

Alternate viewpoint. Fast way to multiply and evaluate polynomials.

coefficient representation

A(x) = a_0 + a_1x + a_2x^2 + \dots + a_{n-1}x^{n-1}\
B(x) = b_0 + b_1x + b_2x^2 + \dots + b_{n-1}x^{n-1}
Add. $O(n)$ arithmetic operations.
A(x) + B(x) = (a_0+b_0) + (a_1 + b_1)x + \dots + (a_{n-1} + b_{n-1})x^{n-1}
Evaluate. $O(n)$ using Horner's method.
A(x) = a_0 + x(a_1 + x(a_2 + \dots + x(a_{n-2} + x(a_{n-1}))\dots))
Multiply (convolve): $O(n^2)$ using brute force
A(x) \times B(x) = \sum_{i=0}^{2n-2}c_ix^i,\qquad where, c_i = \sum_{j=0}^{i}a_jb_{i-j}

Point-Value Representation

Fundamental theorem of algebra. [Gauss, PhD thesis]

A degree n polynomial with complex coefficients has exactly n complex roots.
Corollary. A degree n-1 polynomial A(x) is uniquely specified by its evaluation at n distinct values of x.

A(x): (x_0, y_0), \dots , (x_{n-1}, y_{n-1})\
B(x): (x_0, z_0), \dots , (x_{n-1}, z_{n-1})
Add: $O(n)$ arithmetic operations.
A(x) + B(x): (x_0, y_0 + z_0), \dots , (x_{n-1}, y_{n-1} + z_{n-1})
Evaluate: $O(n^2)$ using Lagrange's formula.
A(x) = \sum_{k=0}^{n-1}y_k\frac{\prod_{j \neq k}(x-x_j)}{\prod_{j\neq k}(x_k - x_j)}
Multiply (convolve). $O(n)$, but need 2n-1 points
A(x) \times B(x): (x_0, y_0 \times z_0), \dots , (x_{n-1}, y_{n-1} \times z_{n-1})

Converting Between Two Representations

Coefficient => point-value: $O(n^2)$ for matrix-vector multiply (or $O(n)$ Horner's).
Point-value => coefficient: $O(n^3)$ for Gaussian elimination.

Improve? => Divide and conquer

Divide. Break polynomial up into even and odd powers
A(x) &= a_0 + a_1x + a_2x^2 + a_3x^3 + a_4x^4 + a_5x^5 + a_6x^6 + a_7x^7\
A_{even}(x) &= a_0 + a_2x^2 + a_4x^4 + a_6x^6\
A_{odd}(x) &= a_1x + a_3x^3 + a_5x^5 + a_7x^7\
A(x) &= A_{even}(x^2) + x A_{odd}(x^2)\
A(-x) &= A_{even}(x^2) - x A_{odd}(x^2)
Intuition: Choose four complex points to be ±1, ±i.
A(1) &= A_{even}(1) + A_{odd}(1)\
A(-1) &= A_{even}(1) -  A_{odd}(1)\
A(i) &= A_{even}(-1) + i A_{odd}(-1)\
A(-i) &= A_{even}(-1) - i A_{odd}(-1).

Can evaluate polynomial of degree $\leq$ n at 4 points by evaluating two polynomials of degree $\leq \frac{2}{n}$ at 2 points.

Fast Fourier Transform (FFT)

Roots of Unity:

  • Def:  An $n_{th}$ root of unity is a complex number x such that $x_n$ = 1.
  • Fact. The $n_{th}$ roots of unity are: $\omega_0, \omega_1, \dots , \omega_{n-1}$ where $\omega = e^{\frac{2\pi i}{n}}$
  • Pf. $(\omega^k)^n = (e^{2\pi i k /n} ) ^n = (e^{\pi i}) ^{2k} = (-1)^{2k} = 1$
  • Fact. The $\frac{n}{2}_{th}$ roots of unity are: $v^0, v^1, \dots, v^{n/2-1}$ where $v = \omega^2 = e^{4\pi i / n}$.

Goal. Evaluate a degree n-1 polynomial $A(x) = a_0 + a_1x + a_2x^2 + \dots + a_{n-1}x^{n-1}$ at its $n_{th}$ roots of unity: $\omega_0, \omega_1, \dots , \omega_{n-1}$

Divide: Break up polynomial into even and odd powers.
A_{even}(x) &= a_0 + a_2x^1 + a_4x^2 + \dots +  a_{n-2}x^{n/2-2}\
A_{odd}(x) &= a_1x + a_3x^1 + a_5x^2 + \dots + a_{n-1}x^{n/2-1}\
A(x) &= A_{even}(x^2) + x A_{odd}(x^2)
Conquer: Evaluate $A_{even}(x)$ and $A_{odd}(x)$ at the $\frac{n}{2}_{th}$ roots of unity: $v^0, v^1, \dots, v^{n/2-1}$.


  • $A(\omega^k) \qquad= A_{even}(v^k) + \omega^kA_{odd}(v^k), \qquad 0 \leq k \leq n/2$
  • $A(\omega^{k+1/2n}) = A_{even}(v^k) - \omega^kA_{odd}(v^k), \qquad 0 \leq k \leq n/2$

Running Time:
T(n) &= 2T(\frac{n}{2})  + \Theta(n) \
&= \Theta(nlogn)