整数对查找

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时间最优的解法

时间复杂度为O（N）
空间复杂度为O（N）

package 整数对查找;import java.util.HashMap;/** * Created by h on 16-12-17. */public class Solution {    public int countPairs(int[] A, int n, int sum) {        // write code here        HashMap<Integer, Integer> hms = new HashMap<Integer, Integer>();        HashMap<Integer, Boolean> visited = new HashMap<Integer, Boolean>();        for (int i :                A) {            if (hms.containsKey(i)) {                hms.put(i, hms.get(i)+1);            } else {                hms.put(i, 1);                visited.put(i, false);            }        }        int ans = 0;        for (Integer i:             hms.keySet()) {            if(!visited.get(i)) {                int another = sum - i;                if (another == i) {                    int num = hms.get(i);                    ans += (num - 1) * num / 2;                } else {                    if (hms.containsKey(another)) {                        ans += hms.get(i) * hms.get(another);                        visited.put(another,true);                    } else {                        continue;                    }                }            }            visited.put(i, true);        }        return ans;    }    public static void main(String[] args) {        int[] A = {11,7,7,6,14,2,14,15,2,1,2,12,13,9,8,15,13,8,10,11,14,10,2,9,4,9,3,7,6,10,15,4,7,6,15,3,9,13,5,2,6,10,10,1,12,4,3,3,8,8,1,4,7,11,13,5,13,15,4,3,1,11,6,11,9,9,11,15,12,10,13,3,11,4,8,9,7,3,13,9,11,3,2,11,10,1,4,2,3,3,14,11,5,10,1,14,8,1,11,3,1,9,14,6,1,7,15,10,14,6,4,12,11};        int n = A.length;        int sum = 16;        System.out.println(new Solution().countPairs(A, n, sum));    }}

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