Documenting LeetCode solving.
Q120
Medium. DP
class Solution:
def findTargetSumWays(self, nums: List[int], target: int) -> int:
dp = {} # (index, total) -> number of ways
def backtrack(i, total):
if i == len(nums):
return 1 if total == target else 0
if (i, total) in dp:
return dp[(i, total)]
# Number of ways to build the expression when index == i,
# total == total
dp[(i, total)] = (backtrack(i + 1, total + nums[i]) +
backtrack(i + 1, total - nums[i]))
return dp[(i, total)]
return backtrack(0, 0)
R22
150. Evaluate Reverse Polish Notation
Medium. Stack
class Solution:
def evalRPN(self, tokens: List[str]) -> int:
stack = []
for c in tokens:
if c == '+':
stack.append(stack.pop() + stack.pop())
elif c == '-':
b, a = stack.pop(), stack.pop()
stack.append(a - b)
elif c == '*':
stack.append(stack.pop() * stack.pop())
elif c == '/':
b, a = stack.pop(), stack.pop()
stack.append(int(a / b))
else:
stack.append(int(c))
return stack[0]