这是leetcode中的3sum问题
这是解决方案之一：
/**
* step 1: nums.length<3; return [];
* step 2: every element can only be used once // var used=[]; if(used[i]){ continue ;}else{ used[i]=true; excute; }
* step 3: [-1, -1, 2] [-1, 2, -1] [2, -1, -1], the same, // Elements in a solution (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
* step 4: backTracking function
*/
var threeSum = function(nums) {
if(nums.length<3) { return []; } // step 1
nums.sort(function(val1,val2) {
return val1>val2?1:val1<val2?-1:0;
});
// console.log(nums);
var solution = [];
var result = [];
var used = [];
var backTracking = function(m, n){ // step 4
if(m==n){
// console.log(solution);
if(solution[0]+solution[1]+solution[2]===0){
return result.push(solution.slice(0));
}
return false;
}else{
var last_num;
for(var i=0; i<nums.length; i++){
if(used[i]) { continue; } // step 2
if(last_num==nums[i]) { continue; }
if(m>0 && solution[m-1]>nums[i]) { continue; } // step 3
used[i] = true;
last_num = nums[i];
solution[m] = nums[i];
arguments.callee(m+1, n);
used[i] = false;
}
}
}
backTracking(0, 3);
return result;
};
这是我尝试写的：
var threeSum = function(ns) {
// < 3, empty
if(ns.length < 3) return [];
ns.sort((a, b) => {
return a-b;
});
let used = new Array(ns.length).fill(false);
bt(res=[], sol=[], used, ns, 0, 3);
return res;
};
var bt = function(res, sol, used, ns, m, n) {
//console.log(res, sol, used, ns, m, n)
if(m === n) {
if(sol.length === 3 && sol[0]+sol[1]+sol[2] === 0) {
//console.log('push')
return res.push(sol);
}
//console.log('re false');
return false;
} else {
let lastNum;
for(let i=0; i<ns.length; i++) {
// each num used once only
if(used[i]) continue;
// not next each other
if(lastNum === ns[i]) continue;
// 1. [-1, -1, 2] [-1, 2, -1] [2, -1, -1], the same
// 2. ns[i] >= sol[m-1], must be
// 3. ns[i] < sol[m-1], con
if(m>0 && sol[m-1] > ns[i]) continue;
// hash
used[i] = true;
// prev #
lastNum = ns[i];
// into sol
let sol1 = sol.concat(ns[i]);
// recur
bt(res, sol1, used, ns, m+1, n);
// ????
used[i] = false;
}
}
}
在此测试用例中失败：
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答案 0 :(得分：0)
每次您要使用一个元素，直到使用为止。因此，您正在运行n ^ 3解决方案。
您遇到“超过时限”的情况包含约3000个元素。 因此，在这种情况下，您算法的复杂度约为3000 * 3000 * 3000 = 27 * 10 ^ 9，这太高了，无法被接受。
尝试将复杂度降低为O（n ^ 2）或O（n ^ 2log（n））
答案 1 :(得分：0)
您需要一种新算法；这是低效率的。
将列表按升序排列。您将嵌套循环以遍历非正数和非负数部分。
找到第一个0；将该位置称为“中心”。
solution = set() # empty set
for neg in (center, 0, -1): # iterate back to the list beginning
for pos in (center+1, len(ns), 1): # iterate to end of list
need = ns(neg) + ns(pos)
if need in ns:
# sort elements and add to solution set
solution.add( sorted(ns(neg), ns(pos), need) )
现在，我会为您解决问题。假设您有一个起始数组ns = [-20, -5, 0, 1, 4, 16, 20]。您很快就会在neg=0, pos=2, need=4找到一个解决方案，并将（-5，1，4）添加到您的解决方案集中。然后，您将迭代到neg=0, pos=3，它将在排序之前再次以（-5，4，1）查找解决方案。