LeetCode
LeetCode 282 Expression Add Operators - Hard
282. Expression Add Operators -- Hard
282. Expression Add Operators — Hard
Problem
- Expression Add Operators -- Hard
Given a string that contains only digits 0-9 and a target value, return all possibilities to add binary operators (not unary) +, -, or * between the digits so they evaluate to the target value.
Example 1:
Input: num = "123", target = 6 Output: ["1+2+3", "123"]
Example 2:
Input: num = "232", target = 8 Output: ["23+2", "2+32"]
Example 3:
Input: num = "105", target = 5 Output: ["1*0+5","10-5"]
Example 4:
Input: num = "00", target = 0 Output: ["0+0", "0-0", "0*0"]
Example 5:
Input: num = "3456237490", target = 9191 Output: []
Constraints:
0 <= num.length <= 10 num only contain digits.
Solution
class Solution:
def addOperators(self, num: str, target: int) -> List[str]:
#dfs 每一层递归都进行加减乘除的尝试，回溯
def dfs(idx, tmp, tot, last, res):
if idx == len(num):
if tot == target:
res.append(tmp)
return
for i in range(idx, len(num)):
x = int(num[idx: i + 1])
if idx == 0:
dfs(i + 1, str(x), x, x, res)
else:
dfs(i + 1, tmp + "+" + str(x), tot + x, x, res)
dfs(i + 1, tmp + "-" + str(x), tot - x, -x, res)
dfs(i + 1, tmp + "*" + str(x), tot - last + last * x, last * x, res)
if x == 0:
break
res = []
dfs(0, "", 0, 0, res)
return res