3Sum Closest

http://discuss.leetcode.com/questions/197/3sum-closest

Given an array S of n integers, find three integers in S such that the sum is closest to a given number, target. Return the sum of the three integers. You may assume that each input would have exactly one solution.

For example, given array S = {-1 2 1 -4}, and target = 1.

    The sum that is closest to the target is 2. (-1 + 2 + 1 = 2).

public int threeSumClosest(int[] num, int target) {
    if (num.length<3) { // if less than three items then return 0
        return Integer.MAX_VALUE;
    }
    Arrays.sort(num);
    int res = num[0]+num[1]+num[2];
    for (int i=0; i<num.length-2; ++i) {
        if (i>0 && num[i]==num[i-1])  continue;
        int start = i+1, end = num.length-1;
        while (start<end) {
            int sum = num[i] + num[start] + num[end];
            if (Math.abs(sum-target) < Math.abs(res-target)) {
                res = sum;
            }
            if (sum == target) {
                return res;
            } else if (sum < target) {
                start++;
            } else {
                end--;
            }
        }//end-while
    }
    return res;
}

1. 固定一个值，左右两个指针在这个值的后面

2. 遇到相同的值，continue，不用再次计算