[LeetCode] 170. Two Sum III - Data structure design

Design a data structure that accepts a stream of integers and checks if it has a pair of integers that sum up to a particular value.

Implement the TwoSum class:

• TwoSum() Initializes the TwoSum object, with an empty array initially.
• void add(int number) Adds number to the data structure.
• boolean find(int value) Returns true if there exists any pair of numbers whose sum is equal to value, otherwise, it returns false.

Example 1:

Input
["TwoSum", "add", "add", "add", "find", "find"]
[[], [1], [3], [5], [4], [7]]
Output
[null, null, null, null, true, false]

Explanation
TwoSum twoSum = new TwoSum();
twoSum.add(1);   // [] --> [1]
twoSum.add(3);   // [1] --> [1,3]
twoSum.add(5);   // [1,3] --> [1,3,5]
twoSum.find(4);  // 1 + 3 = 4, return true
twoSum.find(7);  // No two integers sum up to 7, return false

Constraints:

• -105 <= number <= 105
• -231 <= value <= 231 - 1
• At most 104 calls will be made to add and find.

两数之和 III - 数据结构设计。

Two Sum的变种，是一道设计题。实现两个函数 add() 和 find()。

思路是用 hashmap 存，遍历 input 的时候可以找 target - num1 是否存在于 hashmap，需要记得判断当 num1 + num2 === sum 时，若 num1 === num2，看 hashmap 里面 num 的 value 是否大于1。

Java实现

class TwoSum {
    private HashMap<Integer, Integer> map;
    private List<Integer> list;
    /** Initialize your data structure here. */
    public TwoSum() {
        map = new HashMap<>();
        list = new ArrayList<>();
    }

    /** Add the number to an internal data structure.. */
    public void add(int number) {
        if (!map.containsKey(number)) {
            map.put(number, 1);
            list.add(number);
        } else {
            map.put(number, map.get(number) + 1);
        }
    }

    /** Find if there exists any pair of numbers which sum is equal to the value. */
    public boolean find(int value) {
        for (int num1 : list) {
            int num2 = value - num1;
            if ((num1 == num2 && map.get(num1) > 1) || (num1 != num2 && map.containsKey(num2))) {
                return true;
            }
        }
        return false;
    }
}

/**
 * Your TwoSum object will be instantiated and called as such:
 * TwoSum obj = new TwoSum();
 * obj.add(number);
 * boolean param_2 = obj.find(value);
 */

 

JavaScript实现

/**
 * Initialize your data structure here.
 */
var TwoSum = function() {
    this.map = {};
};

/**
 * Add the number to an internal data structure..
 * @param {number} number
 * @return {void}
 */
TwoSum.prototype.add = function(number) {
    if (this.map[number]) {
        this.map[number]++;
    } else {
        this.map[number] = 1;
    }
};

/**
 * Find if there exists any pair of numbers which sum is equal to the value.
 * @param {number} value
 * @return {boolean}
 */
TwoSum.prototype.find = function(value) {
    for (var key in this.map) {
        var num1 = key;
        var num2 = value - key;
        if (this.map[num2]) {
            if (num1 != num2 || this.map[num2] > 1) {
                return true;
            }
        }
    }
    return false;
};

/**
 * Your TwoSum object will be instantiated and called as such:
 * var obj = new TwoSum()
 * obj.add(number)
 * var param_2 = obj.find(value)
 */

 

two sum题目总结

LeetCode 题目总结