I 非排好序，最快O（n）， 用hashmap解決

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public class Solution {public int[] twoSum(int[] numbers, int target) {int[] res = new int[2];HashMap<Integer, Integer> hm = new HashMap<Integer, Integer>();if(numbers==null || numbers.length<2) return res;for(int i=0; i<numbers.length; i++){if(!hm.containsKey(target-numbers[i])){hm.put(numbers[i], i);}else{res[0] = hm.get(target-numbers[i])+1;res[1] = i+1;break;}}return res;} }

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II 排好序數組，2 pointers解決

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public class Solution {public int[] twoSum(int[] numbers, int target) {int[] res = new int[2];int sum = 0, low =0, high = numbers.length-1;if(numbers==null || numbers.length<2) return res;while(low<high){sum = numbers[low]+numbers[high];if(sum==target){res[0] = low+1;res[1] = high+1;break;}else if(sum>target){high--;}elselow++;}return res;} }

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III

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Two Sum III - Data structure design (LeetCode HashMap)

Question:??Design and implement a TwoSum class. It should support the following operations: add and find.

add - Add the number to an internal data structure.
find - Find if there exists any pair of numbers which sum is equal to the value.

For example,
add(1); add(3); add(5);
find(4) -> true
find(7) -> false

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注意其中如果target ==2 elements in set怎么做判斷，見 ref ：http://shanjiaxin.blogspot.com/2015/01/two-sum-iii-data-structure-design.html

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copy其code：

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import java.util.HashMap; import java.util.Map;// case: add(0) -> find(0) -> should be false public class TwoSumIII {Map<Integer, Integer> dict = new HashMap<Integer, Integer>();public void add(int number) {if (dict.containsKey(number)) {dict.put(number, dict.get(number) + 1);} else {dict.put(number, 1);}}public boolean find(int value) {for (Integer key : dict.keySet()) {<span style="color:#ff0000;"> if (value - key == key) {if (dict.get(key) >= 2) {return true;</span>}} else if (dict.containsKey(value - key)) {return true;}}return false;}public static void main(String[] args) {TwoSumIII util = new TwoSumIII();util.add(1);util.add(3);util.add(5);System.out.println(util.find(4));System.out.println(util.find(7));}}

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轉載于:https://www.cnblogs.com/jiajiaxingxing/p/4391502.html

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