[LintCode] Submatrix Sum

Problem

Given an integer matrix, find a submatrix where the sum of numbers is zero. Your code should return the coordinate of the left-up and right-down number.

Example

Given matrix

[
  [1 ,5 ,7],
  [3 ,7 ,-8],
  [4 ,-8 ,9],
]

return [(1,1), (2,2)]

Challenge

O(n3) time.

Note

原理是這樣的：
先對矩陣matrix的每一個點到左頂點之間的子矩陣求和，存在新矩陣sum上。注意，sum[i+1][j+1]表明的是matrix[0][0]到matrix[i][j]的子矩陣求和。以下所示：

Given matrix[m][n]
------------------
[
  [1 ,5 ,7],
  [3 ,7 ,-8],
  [4 ,-8 ,9],
]
Get sum[m+1][n+1]
-----------------
0  0  0  0
0  1  6  13
0  4  16 15
0  8  12 20

而後，咱們進行這樣的操做，從0開始，肯定兩行i1、i2，再將第k列的sum[i1][k]和sum[i2][k]相減，就獲得matrix[i1][0]到matrix[i2][k-1]的子矩陣（i2-i1行，k列）求和diff，存入map。仍是在這兩行，假設計算到第j列，獲得了相等的和diff。說明從i1到i2行，從k到j列的子矩陣求和爲0，即至關於兩個平行放置的矩形，若左邊的值爲diff，左邊與右邊之和也是diff，那麼右邊的值必定爲0。下面是map中存放操做的例子：

diff-j chart
------------
diff    j

1       1       i1 = 0, i2 = 1
6       2
13      3
4       1       i1 = 0, i2 = 2
16      2
15      3
8       1       i1 = 0, i2 = 3
12      2
20      3
3       1       i1 = 1, i2 = 2
10      2
2       3
------------------------------
(above res has no same pair in same region)
7       1       i1 = 1, i2 = 3
6       2       
7       3       diff-j pair exists in map

res[0][0] = i1 = 1, res[0][1] = map.get(diff) = 1,
res[1][0] = i2 - 1 = 3 - 1 = 2, res[1][1] = j = 2,

so res = [(1, 1), (2, 2)]

Solution

public class Solution {
    public int[][] submatrixSum(int[][] matrix) {
        int m = matrix.length, n = matrix[0].length;
        int[][] sum = new int[m+1][n+1];
        int[][] res = new int[2][2];
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                sum[i+1][j+1] = matrix[i][j] + sum[i][j+1] + sum[i+1][j] - sum[i][j];
            }
        }
        for (int i1 = 0; i1 < m; i1++) {
            for (int i2 = i1+1; i2 <= m; i2++) {
                Map<Integer, Integer> map = new HashMap<Integer, Integer>();
                for (int j = 0; j <= n; j++) {
                    int diff = sum[i2][j] - sum[i1][j];
                    if (map.containsKey(diff)) {
                        res[0][0] = i1; res[0][1] = map.get(diff);
                        res[1][0] = i2-1; res[1][1] = j-1;
                        return res;
                    }
                    else map.put(diff, j);
                }
            }
        }
        return res;
    }
}