Say you have an array for which the ith element is the price of a given stock on day i.
Design an algorithm to find the maximum profit. You may complete at most k
transactions.
Example
Given prices = [4,4,6,1,1,4,2,5]
,
and k = 2
, return 6
.
状态转移方程为:
local[i][j] = max(global[i-1][j-1] + max(diff,0), local[i-1][j]+diff)
global[i][j] = max(local[i][j], global[i-1][j])
因为在计算local[i][j]时,i-1,交易了j次,并且位置i因为和第j次交易相邻,可以并入第j次交易的情况,所以需要使用两个数组,一个数组local记录以i为最后一次卖出时,最大收益,和一个数组global,记录0到i范围内整体最大收益。这样local可以用来计算之前提到的交易合并问题。
以下方法运行内存超出规定大小。
class Solution {
/**
* @param k: An integer
* @param prices: Given an integer array
* @return: Maximum profit
*/
public int maxProfit(int k, int[] prices) {
if(prices == null || prices.length == 0) return 0;
int[][] local = new int[prices.length][k + 1];
int[][] global = new int[prices.length][k + 1];
int res = 0;
for(int j = 1; j <= k; j++) {
for(int i = 1; i < prices.length; i++) {
int diff = prices[i] - prices[i - 1];
local[i][j] = Math.max(local[i - 1][j] + diff,
global[i - 1][j - 1] + Math.max(0, diff));
global[i][j] = Math.max(global[i - 1][j], local[i][j]);
}
}
return global[prices.length - 1][k];
}
};
考虑使用两个滚动数组代替。滚动数组要确保覆盖后的值不会被用到。因此又引入了临时变量tmp。
现在内存符合要求,但是超时。继续改。
class Solution {
/**
* @param k: An integer
* @param prices: Given an integer array
* @return: Maximum profit
*/
public int maxProfit(int k, int[] prices) {
if(prices == null || prices.length == 0) return 0;
int[] global = new int[prices.length];
int[] local = new int[prices.length];
int res = 0;
for(int j = 1; j <= k; j++) {
int tmp = global[0];
for(int i = 1; i < prices.length; i++) {
int diff = prices[i] - prices[i - 1];
local[i] = Math.max(local[i - 1] + diff,
tmp + Math.max(0, diff));
tmp = global[i];
global[i] = Math.max(global[i - 1], local[i]);
if(global[i] > res) res = global[i];
}
}
return res;
}
};
当i小于j时,例如3个交易日需要进行4次交易的情况,可以以排除,所以给i加上限制。i>=j. 通过测试。
class Solution {
/**
* @param k: An integer
* @param prices: Given an integer array
* @return: Maximum profit
*/
public int maxProfit(int k, int[] prices) {
if(prices == null || prices.length == 0) return 0;
int[] global = new int[prices.length];
int[] local = new int[prices.length];
//Memory Limit Exceeded,改用两个一维滚动数组
int res = 0;
for(int j = 1; j <= k; j++) {
int tmp = global[0];
for(int i = j; i < prices.length; i++) {
int diff = prices[i] - prices[i - 1];
local[i] = Math.max(local[i - 1] + diff,
tmp + Math.max(0, diff));
tmp = global[i];
global[i] = Math.max(global[i - 1], local[i]);
if(global[i] > res) res = global[i];
}
}
return res;
}
};