两数之和.md

Two-Sum

给定一个整数数组，给定一个目标值，返回数组里面相加值为目标值的数字的索引。你得假设每个目标值都会有一个解决方案。

解决方案：

const nums = [2, 7, 11, 15],
    target = 9;
const twoSum = function(nums, target) {
    const length = nums.length,
        num = [];
    for(let i = 0; i < length-1;i++){
        for(let j = i+1; j< length; j++){
            if(nums[i]+ nums[j] === target){
                num.push(i , j);
            }
        }
    }
    return num;
};
twoSum (nums, target);

✔ Accepted
✔ 29/29 cases passed (248 ms)
✔ Your runtime beats 28.7 % of javascript submissions
✔ Your memory usage beats 59.4 % of javascript submissions (34.6 MB)

优化后：

const nums = [2, 7, 11, 15],
    target = 9;
const twoSum = function(nums, target) {
    const length = nums.length,
        num = [];
    for(let i = 0; i < length-1;i++){
        let result = target - nums[i];
        for(let j = i+1; j< length; j++){
            if(nums[j] === result){
                num.push(i , j);
            }
        }
    }
    return num;
};
twoSum (nums, target);

✔ Accepted
✔ 29/29 cases passed (220 ms)
✔ Your runtime beats 38.38 % of javascript submissions
✔ Your memory usage beats 24.71 % of javascript submissions (35 MB)

再次优化

const nums = [2, 7, 11, 15],
    target = 9;
const twoSum = function(nums, target) {
  const ret = [];
  const exist = {};
  for(let i = 0; i < nums.length; i++){
    if(typeof(exist[target - nums[i]]) !== 'undefined'){
        ret.push(exist[target - nums[i]]);
        ret.push(i);
        return ret;
    }
    exist[nums[i]] = i;
  }
};

✔ Accepted
✔ 29/29 cases passed (88 ms)
✔ Your runtime beats 99.31 % of javascript submissions
✔ Your memory usage beats 82.23 % of javascript submissions (34.3 MB)