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Merge pull request soapyigu#196 from soapyigu/DFS
[DFS] Add a solution to Expression Add Operators
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/** | ||
* Question Link: https://leetcode.com/problems/expression-add-operators/ | ||
* Primary idea: Classic Depth-first Search, terminates at when position encounters the | ||
* length of the string num, add to result when eval is equal to target | ||
* | ||
* Note: | ||
* 1. String cast to Integer will make character loss, e.g. "05" -> 5 | ||
* 2. Multiplication's priority is higher than addiction | ||
* | ||
* Time Complexity: O(n!), Space Complexity: O(n) | ||
* | ||
*/ | ||
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class ExpressionAddOperators { | ||
func addOperators(_ num: String, _ target: Int) -> [String] { | ||
var res = [String]() | ||
let numChars = Array(num.characters) | ||
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guard numChars.count > 0 else { | ||
return res | ||
} | ||
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dfs(&res, "", numChars, target, 0, 0, 0) | ||
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return res | ||
} | ||
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private func dfs(_ res: inout [String], _ temp: String, _ numChars: [Character], _ target: Int, _ pos: Int, _ eval: Int, _ mul: Int) { | ||
if pos == numChars.count { | ||
if eval == target { | ||
res.append(temp) | ||
} | ||
return | ||
} | ||
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for i in pos ..< numChars.count { | ||
if i != pos && numChars[pos] == "0" { | ||
break | ||
} | ||
let curt = Int(String(numChars[pos ..< i + 1]))! | ||
if pos == 0 { | ||
dfs(&res, temp + String(curt), numChars, target, i + 1, curt, curt) | ||
} else { | ||
dfs(&res, temp + "+" + String(curt), numChars, target, i + 1, eval + curt, curt) | ||
dfs(&res, temp + "-" + String(curt), numChars, target, i + 1, eval - curt, -curt) | ||
dfs(&res, temp + "*" + String(curt), numChars, target, i + 1, eval - mul + mul * curt, mul * curt) | ||
} | ||
} | ||
} | ||
} |