[CareerCup] 18.12 Largest Sum Submatrix 和最大的子矩阵 - Grandyang - 博客园


[CareerCup] 18.12 Largest Sum Submatrix 和最大的子矩阵 - Grandyang - 博客园
18.12 Given an NxN matrix of positive and negative integers, write code to find the submatrix with the largest possible sum.
 https://github.com/careercup/CtCI-6th-Edition/tree/master/Java/Ch%2017.%20Hard/Q17_24_Max_Submatrix
这道求和最大的子矩阵,跟LeetCode上的Maximum Size Subarray Sum Equals kMaximum Subarray很类似。

If we have R rows and C columns, we can solve it in O(R^2C) time.
public class SubMatrix {
private int row1, row2, col1, col2, sum;
}
public class Range {
public int start, end, sum;
}

public static SubMatrix getMaxMatrix(int[][] matrix) {
int rowCount = matrix.length;
int colCount = matrix[0].length;

SubMatrix best = null;

for (int rowStart = 0; rowStart < rowCount; rowStart++) {
int[] partialSum = new int[colCount];

for (int rowEnd = rowStart; rowEnd < rowCount; rowEnd++) {
/* Add values at row rowEnd. */
for (int i = 0; i < colCount; i++) {
partialSum[i] += matrix[rowEnd][i];
}

Range bestRange = maxSubArray(partialSum, colCount);
if (best == null || best.getSum() < bestRange.sum) {
best = new SubMatrix(rowStart, bestRange.start, rowEnd, bestRange.end, bestRange.sum);
}
}
}
return best;
}

public static Range maxSubArray(int[] array, int N) {
Range best = null;
int start = 0;
int sum = 0;

for (int i = 0; i < N; i++) {
sum += array[i];
if (best == null || sum > best.sum) {
best = new Range(start, i, sum);
}

/* If running_sum is < 0 no point in trying to continue the
* series. Reset. */
if (sum < 0) {
start = i + 1;
sum = 0;
}
}
return best;
}

这道题不建议使用brute force的方法,因为实在是不高效,我们需要借鉴上面LeetCode中的建立累计和矩阵的思路,我们先来看这道题的第一种解法,由于建立好累计和矩阵,那么我们通过给定了矩阵的左上和右下两个顶点的坐标可以在O(1)的时间内快速的求出矩阵和,所以我们要做的就是遍历矩阵中所有的子矩阵,然后比较其矩阵和,返回最大的即可,时间复杂度为O(n4)。
public static SubMatrix getMaxMatrix(int[][] matrix) {
SubMatrix best = null;
int rowCount = matrix.length;
int columnCount = matrix[0].length;
int[][] sumThrough = precomputeSums(matrix);

for (int row1 = 0; row1 < rowCount; row1++) {
for (int row2 = row1; row2 < rowCount; row2++) {
for (int col1 = 0; col1 < columnCount; col1++) {
for (int col2 = col1; col2 < columnCount; col2++) {
int sum = sum(sumThrough, row1, col1, row2, col2);
if (best == null || best.getSum() < sum) {
best = new SubMatrix(row1, col1, row2, col2, sum);
}
}
}
}
}
return best;
}

private static int[][] precomputeSums(int[][] matrix) {
int[][] sumThrough = new int[matrix.length][matrix[0].length];
for (int r = 0; r < matrix.length; r++) {
for (int c = 0; c < matrix[0].length; c++) {
int left = c > 0 ? sumThrough[r][c - 1] : 0;
int top = r > 0 ? sumThrough[r - 1][c] : 0;
int overlap = r > 0 && c > 0 ? sumThrough[r - 1][c - 1] : 0;
sumThrough[r][c] = left + top - overlap + matrix[r][c];
}
}
return sumThrough;
}

private static int sum(int[][] sumThrough, int r1, int c1, int r2, int c2) {
int topAndLeft = r1 > 0 && c1 > 0 ? sumThrough[r1 - 1][c1 - 1] : 0;
int left = c1 > 0 ? sumThrough[r2][c1 - 1] : 0;
int top = r1 > 0 ? sumThrough[r1 - 1][c2] : 0;
int full = sumThrough[r2][c2];
return full - left - top + topAndLeft;
}

O(N^6)
public static SubMatrix getMaxMatrix(int[][] matrix) {
int rowCount = matrix.length;
int columnCount = matrix[0].length;
SubMatrix best = null;
for (int row1 = 0; row1 < rowCount; row1++) {
for (int row2 = row1; row2 < rowCount; row2++) {
for (int col1 = 0; col1 < columnCount; col1++) {
for (int col2 = col1; col2 < columnCount; col2++) {
int sum = sum(matrix, row1, col1, row2, col2);
if (best == null || best.getSum() < sum) {
best = new SubMatrix(row1, col1, row2, col2, sum);
}
}
}
}
}
return best;
}

private static int sum(int[][] matrix, int row1, int col1, int row2, int col2) {
int sum = 0;
for (int r = row1; r <= row2; r++) {
for (int c = col1; c <= col2; c++) {
sum += matrix[r][c];
}
}
return sum;
}

Read full article from [CareerCup] 18.12 Largest Sum Submatrix 和最大的子矩阵 - Grandyang - 博客园

Labels

LeetCode (1432) GeeksforGeeks (1122) LeetCode - Review (1067) Review (882) Algorithm (668) to-do (609) Classic Algorithm (270) Google Interview (237) Classic Interview (222) Dynamic Programming (220) DP (186) Bit Algorithms (145) POJ (141) Math (137) Tree (132) LeetCode - Phone (129) EPI (122) Cracking Coding Interview (119) DFS (115) Difficult Algorithm (115) Lintcode (115) Different Solutions (110) Smart Algorithm (104) Binary Search (96) BFS (91) HackerRank (90) Binary Tree (86) Hard (79) Two Pointers (78) Stack (76) Company-Facebook (75) BST (72) Graph Algorithm (72) Time Complexity (69) Greedy Algorithm (68) Interval (63) Company - Google (62) Geometry Algorithm (61) Interview Corner (61) LeetCode - Extended (61) Union-Find (60) Trie (58) Advanced Data Structure (56) List (56) Priority Queue (53) Codility (52) ComProGuide (50) LeetCode Hard (50) Matrix (50) Bisection (48) Segment Tree (48) Sliding Window (48) USACO (46) Space Optimization (45) Company-Airbnb (41) Greedy (41) Mathematical Algorithm (41) Tree - Post-Order (41) ACM-ICPC (40) Algorithm Interview (40) Data Structure Design (40) Graph (40) Backtracking (39) Data Structure (39) Jobdu (39) Random (39) Codeforces (38) Knapsack (38) LeetCode - DP (38) Recursive Algorithm (38) String Algorithm (38) TopCoder (38) Sort (37) Introduction to Algorithms (36) Pre-Sort (36) Beauty of Programming (35) Must Known (34) Binary Search Tree (33) Follow Up (33) prismoskills (33) Palindrome (32) Permutation (31) Array (30) Google Code Jam (30) HDU (30) Array O(N) (29) Logic Thinking (29) Monotonic Stack (29) Puzzles (29) Code - Detail (27) Company-Zenefits (27) Microsoft 100 - July (27) Queue (27) Binary Indexed Trees (26) TreeMap (26) to-do-must (26) 1point3acres (25) GeeksQuiz (25) Merge Sort (25) Reverse Thinking (25) hihocoder (25) Company - LinkedIn (24) Hash (24) High Frequency (24) Summary (24) Divide and Conquer (23) Proof (23) Game Theory (22) Topological Sort (22) Lintcode - Review (21) Tree - Modification (21) Algorithm Game (20) CareerCup (20) Company - Twitter (20) DFS + Review (20) DP - Relation (20) Brain Teaser (19) DP - Tree (19) Left and Right Array (19) O(N) (19) Sweep Line (19) UVA (19) DP - Bit Masking (18) LeetCode - Thinking (18) KMP (17) LeetCode - TODO (17) Probabilities (17) Simulation (17) String Search (17) Codercareer (16) Company-Uber (16) Iterator (16) Number (16) O(1) Space (16) Shortest Path (16) itint5 (16) DFS+Cache (15) Dijkstra (15) Euclidean GCD (15) Heap (15) LeetCode - Hard (15) Majority (15) Number Theory (15) Rolling Hash (15) Tree Traversal (15) Brute Force (14) Bucket Sort (14) DP - Knapsack (14) DP - Probability (14) Difficult (14) Fast Power Algorithm (14) Pattern (14) Prefix Sum (14) TreeSet (14) Algorithm Videos (13) Amazon Interview (13) Basic Algorithm (13) Codechef (13) Combination (13) Computational Geometry (13) DP - Digit (13) LCA (13) LeetCode - DFS (13) Linked List (13) Long Increasing Sequence(LIS) (13) Math-Divisible (13) Reservoir Sampling (13) mitbbs (13) Algorithm - How To (12) Company - Microsoft (12) DP - Interval (12) DP - Multiple Relation (12) DP - Relation Optimization (12) LeetCode - Classic (12) Level Order Traversal (12) Prime (12) Pruning (12) Reconstruct Tree (12) Thinking (12) X Sum (12) AOJ (11) Bit Mask (11) Company-Snapchat (11) DP - Space Optimization (11) Dequeue (11) Graph DFS (11) MinMax (11) Miscs (11) Princeton (11) Quick Sort (11) Stack - Tree (11) 尺取法 (11) 挑战程序设计竞赛 (11) Coin Change (10) DFS+Backtracking (10) Facebook Hacker Cup (10) Fast Slow Pointers (10) HackerRank Easy (10) Interval Tree (10) Limited Range (10) Matrix - Traverse (10) Monotone Queue (10) SPOJ (10) Starting Point (10) States (10) Stock (10) Theory (10) Tutorialhorizon (10) Kadane - Extended (9) Mathblog (9) Max-Min Flow (9) Maze (9) Median (9) O(32N) (9) Quick Select (9) Stack Overflow (9) System Design (9) Tree - Conversion (9) Use XOR (9) Book Notes (8) Company-Amazon (8) DFS+BFS (8) DP - States (8) Expression (8) Longest Common Subsequence(LCS) (8) One Pass (8) Quadtrees (8) Traversal Once (8) Trie - Suffix (8) 穷竭搜索 (8) Algorithm Problem List (7) All Sub (7) Catalan Number (7) Cycle (7) DP - Cases (7) Facebook Interview (7) Fibonacci Numbers (7) Flood fill (7) Game Nim (7) Graph BFS (7) HackerRank Difficult (7) Hackerearth (7) Inversion (7) Kadane’s Algorithm (7) Manacher (7) Morris Traversal (7) Multiple Data Structures (7) Normalized Key (7) O(XN) (7) Radix Sort (7) Recursion (7) Sampling (7) Suffix Array (7) Tech-Queries (7) Tree - Serialization (7) Tree DP (7) Trie - Bit (7) 蓝桥杯 (7) Algorithm - Brain Teaser (6) BFS - Priority Queue (6) BFS - Unusual (6) Classic Data Structure Impl (6) DP - 2D (6) DP - Monotone Queue (6) DP - Unusual (6) DP-Space Optimization (6) Dutch Flag (6) How To (6) Interviewstreet (6) Knapsack - MultiplePack (6) Local MinMax (6) MST (6) Minimum Spanning Tree (6) Number - Reach (6) Parentheses (6) Pre-Sum (6) Probability (6) Programming Pearls (6) Rabin-Karp (6) Reverse (6) Scan from right (6) Schedule (6) Stream (6) Subset Sum (6) TSP (6) Xpost (6) n00tc0d3r (6) reddit (6) AI (5) Abbreviation (5) Anagram (5) Art Of Programming-July (5) Assumption (5) Bellman Ford (5) Big Data (5) Code - Solid (5) Code Kata (5) Codility-lessons (5) Coding (5) Company - WMware (5) Convex Hull (5) Crazyforcode (5) DFS - Multiple (5) DFS+DP (5) DP - Multi-Dimension (5) DP-Multiple Relation (5) Eulerian Cycle (5) Graph - Unusual (5) Graph Cycle (5) Hash Strategy (5) Immutability (5) Java (5) LogN (5) Manhattan Distance (5) Matrix Chain Multiplication (5) N Queens (5) Pre-Sort: Index (5) Quick Partition (5) Quora (5) Randomized Algorithms (5) Resources (5) Robot (5) SPFA(Shortest Path Faster Algorithm) (5) Shuffle (5) Sieve of Eratosthenes (5) Strongly Connected Components (5) Subarray Sum (5) Sudoku (5) Suffix Tree (5) Swap (5) Threaded (5) Tree - Creation (5) Warshall Floyd (5) Word Search (5) jiuzhang (5)

Popular Posts