leetcode/FindTheDuplicateNumber.java at master · mengli/leetcode · GitHub

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/**
 * Given an unsorted integer array, find the smallest missing positive integer.
 *
 * Example 1:
 *
 * Input: [1,2,0]
 * Output: 3
 * Example 2:
 *
 * Input: [3,4,-1,1]
 * Output: 2
 * Example 3:
 *
 * Input: [7,8,9,11,12]
 * Output: 1
 * Note:
 *
 * Your algorithm should run in O(n) time and uses constant extra space.
 */
classSolution {
publicintfirstMissingPositive(int[] a) {
inti = 0;
while (i < a.length) {
if (a[i] > 0 && a[i] <= a.length && a[i] != a[a[i] - 1]) {
swap(a, i, a[i] - 1);
 } else {
i++;
 }
 }
i = 0;
while (i < a.length && a[i] == i + 1) {
i++;
 }
returni + 1;
 }

privatevoidswap(int[] a, inti, intj) {
intb = a[i];
a[i] = a[j];
a[j] = b;
 }
}