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", "1*2*3"]
Example 2:
Input: num = "232", target = 8
Output: ["2*3+2", "2+3*2"]
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: []
方法： dfs, 递归
class Solution(object):
def addOperators(self, num, target):
def dfs(idx, cal, tot, last, res):
if idx == len(num):
if tot == target:
res.append(cal)
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, cal + "+" + str(x), tot + x, x, res)
dfs(i + 1, cal + "-" + str(x), tot - x, -x, res)
dfs(i + 1, cal + "*" + str(x), tot - last + last * x, last * x, res)
# if x == 0: # not necessary
# break
res = []
dfs(0, "", 0, 0, res)
return res