120. Triangle
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.
Solution: DP
class Solution(object):
def minimumTotal(self, triangle):
"""
:type triangle: List[List[int]]
:rtype: int
"""
sums = [0] * (len(triangle) + 1)
for row in reversed(triangle):
for idx, num in enumerate(row):
sums[idx] = min(sums[idx], sums[idx + 1]) + num
return sums[0]
Lessons:
- Since the lower row is larger than upper row, go from bottom to top.