282. Expression Add Operators
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package LeetCode_282 /** * 282. Expression Add Operators * https://leetcode.com/problems/expression-add-operators/description/ * * 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"] Constraints: 0 <= num.length <= 10 num only contain digits. * */ class Solution { private val result = ArrayList<String>() private var num = "" private var target = 0L /** * solution: dfs, Time complexity:O(4^n), Space complexity:O(n) * */ fun addOperators(num: String, target: Int): List<String> { this.num = num this.target = target.toLong() dfs(0, "", 0, 0) return result } private fun dfs(index: Int, candidate: String, curSum: Long, last: Long) { if (target == curSum && index == num.length) { result.add(candidate) return } for (i in index until num.length) { if (i != index && num[index] == ‘0‘) { break//avoid "01" } val next = num.substring(index, i + 1).toLong() if (index == 0) { //the first must be + dfs(i + 1, next.toString(), next, next) continue } dfs(i + 1, candidate + "+" + next, curSum + next, next) dfs(i + 1, candidate + "-" + next, curSum - next, -next) dfs(i + 1, candidate + "*" + next, curSum - last + last * next, last * next) } } }
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java 282. Expression Add Operators.java
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