标签：style class blog code java ext
Given a triangle, find the minimum path sum from top to bottom. Each step you may move to adjacent numbers on the row below.
For example, given the following triangle
[
[2],
[3,4],
[6,5,7],
[4,1,8,3]
]
The minimum path sum from top to bottom is 11 (i.e., 2 + 3 + 5 + 1 = 11).
Note:
Bonus point if you are able to do this using only O(n) extra space, where n is the total number of rows in the triangle.
public class Solution {
/**Top-down method. Since only O(n) extra memory is allowed, we use a deque to save the intermediate result.
* @author Averill Zheng
* @version 2014-06-23
* @since JDK 1.7
*/
public int minimumTotal(List<List<Integer>> triangle) {
int min = Integer.MAX_VALUE;
int length = triangle.size();
if(length == 0)
return 0;
else if(length == 1)
return triangle.get(0).get(0);
else {
Queue<Integer> sums = new LinkedList<Integer>();
sums.add(triangle.get(0).get(0));
for(int i = 1; i < length; ++i){
List<Integer> aList = triangle.get(i);
int equalIndex = sums.poll();
sums.add(equalIndex + aList.get(i));
int smallerIndex = (i > 1) ? sums.peek(): Math.abs(equalIndex);
for(int j = i - 1; j > 0; --j){
sums.add(Math.min(smallerIndex + aList.get(j), equalIndex + aList.get(j)));
equalIndex = sums.poll();
smallerIndex = sums.peek();
}
sums.add(equalIndex + aList.get(0));
}
while(sums.peek() != null){
min = Math.min(min, sums.poll());
}
}
return min;
}
}
method 2: bottom-up method
public class Solution {
public int minimumTotal(List<List<Integer>> triangle) {
Queue<Integer> sums = new LinkedList<Integer>();
int length = triangle.size();
if(length == 0)
return 0;
else if(length == 1)
return triangle.get(0).get(0);
else {
List<Integer> lastList = triangle.get(length - 1);
for(int i = 0; i < length; ++i)
sums.add(lastList.get(i));
for(int i = length - 2; i > -1; --i){
List<Integer> currentList = triangle.get(i);
for(int j = 0; j < i + 1; ++j){
int equalIndex = sums.poll();
int largerIndex = sums.peek();
sums.add(currentList.get(j) + Math.min(equalIndex, largerIndex));
}
sums.poll();
}
}
return sums.peek();
}
}
method 3. bottom-up , and use List<Integer> as the data structure to save the intermediate result.
public class Solution {
public int minimumTotal(List<List<Integer>> triangle) {
int length = triangle.size();
List<Integer> pathSum = new ArrayList<Integer>();
if(length == 0)
return 0;
pathSum.addAll(triangle.get(length - 1));
for(int i = length - 2; i > -1; --i){
List<Integer> aList = triangle.get(i);
for(int j = 0; j < i + 1; ++j)
pathSum.set(j, Math.min(pathSum.get(j), pathSum.get(j + 1))+ aList.get(j));
}
return pathSum.get(0);
}
}
leetcode--Triangle,布布扣,bubuko.com
标签：style class blog code java ext
原文地址：http://www.cnblogs.com/averillzheng/p/3805657.html
